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---
name: shadcn
description: Manages shadcn components and projects — adding, searching, fixing, debugging, styling, and composing UI. Provides project context, component docs, and usage examples. Applies when working with shadcn/ui, component registries, presets, --preset codes, or any project with a components.json file. Also triggers for "shadcn init", "create an app with --preset", or "switch to --preset".
user-invocable: false
---
# shadcn/ui
A framework for building ui, components and design systems. Components are added as source code to the user's project via the CLI.
> **IMPORTANT:** Run all CLI commands using the project's package runner: `npx shadcn@latest`, `pnpm dlx shadcn@latest`, or `bunx --bun shadcn@latest` — based on the project's `packageManager`. Examples below use `npx shadcn@latest` but substitute the correct runner for the project.
## Current Project Context
```json
!`npx shadcn@latest info --json 2>/dev/null || echo '{"error": "No shadcn project found. Run shadcn init first."}'`
```
The JSON above contains the project config and installed components. Use `npx shadcn@latest docs <component>` to get documentation and example URLs for any component.
## Principles
1. **Use existing components first.** Use `npx shadcn@latest search` to check registries before writing custom UI. Check community registries too.
2. **Compose, don't reinvent.** Settings page = Tabs + Card + form controls. Dashboard = Sidebar + Card + Chart + Table.
3. **Use built-in variants before custom styles.** `variant="outline"`, `size="sm"`, etc.
4. **Use semantic colors.** `bg-primary`, `text-muted-foreground` — never raw values like `bg-blue-500`.
## Critical Rules
These rules are **always enforced**. Each links to a file with Incorrect/Correct code pairs.
### Styling & Tailwind → [styling.md](./rules/styling.md)
- **`className` for layout, not styling.** Never override component colors or typography.
- **No `space-x-*` or `space-y-*`.** Use `flex` with `gap-*`. For vertical stacks, `flex flex-col gap-*`.
- **Use `size-*` when width and height are equal.** `size-10` not `w-10 h-10`.
- **Use `truncate` shorthand.** Not `overflow-hidden text-ellipsis whitespace-nowrap`.
- **No manual `dark:` color overrides.** Use semantic tokens (`bg-background`, `text-muted-foreground`).
- **Use `cn()` for conditional classes.** Don't write manual template literal ternaries.
- **No manual `z-index` on overlay components.** Dialog, Sheet, Popover, etc. handle their own stacking.
### Forms & Inputs → [forms.md](./rules/forms.md)
- **Forms use `FieldGroup` + `Field`.** Never use raw `div` with `space-y-*` or `grid gap-*` for form layout.
- **`InputGroup` uses `InputGroupInput`/`InputGroupTextarea`.** Never raw `Input`/`Textarea` inside `InputGroup`.
- **Buttons inside inputs use `InputGroup` + `InputGroupAddon`.**
- **Option sets (2–7 choices) use `ToggleGroup`.** Don't loop `Button` with manual active state.
- **`FieldSet` + `FieldLegend` for grouping related checkboxes/radios.** Don't use a `div` with a heading.
- **Field validation uses `data-invalid` + `aria-invalid`.** `data-invalid` on `Field`, `aria-invalid` on the control. For disabled: `data-disabled` on `Field`, `disabled` on the control.
### Component Structure → [composition.md](./rules/composition.md)
- **Items always inside their Group.** `SelectItem` → `SelectGroup`. `DropdownMenuItem` → `DropdownMenuGroup`. `CommandItem` → `CommandGroup`.
- **Use `asChild` (radix) or `render` (base) for custom triggers.** Check `base` field from `npx shadcn@latest info`. → [base-vs-radix.md](./rules/base-vs-radix.md)
- **Dialog, Sheet, and Drawer always need a Title.** `DialogTitle`, `SheetTitle`, `DrawerTitle` required for accessibility. Use `className="sr-only"` if visually hidden.
- **Use full Card composition.** `CardHeader`/`CardTitle`/`CardDescription`/`CardContent`/`CardFooter`. Don't dump everything in `CardContent`.
- **Button has no `isPending`/`isLoading`.** Compose with `Spinner` + `data-icon` + `disabled`.
- **`TabsTrigger` must be inside `TabsList`.** Never render triggers directly in `Tabs`.
- **`Avatar` always needs `AvatarFallback`.** For when the image fails to load.
### Use Components, Not Custom Markup → [composition.md](./rules/composition.md)
- **Use existing components before custom markup.** Check if a component exists before writing a styled `div`.
- **Callouts use `Alert`.** Don't build custom styled divs.
- **Empty states use `Empty`.** Don't build custom empty state markup.
- **Toast via `sonner`.** Use `toast()` from `sonner`.
- **Use `Separator`** instead of `<hr>` or `<div className="border-t">`.
- **Use `Skeleton`** for loading placeholders. No custom `animate-pulse` divs.
- **Use `Badge`** instead of custom styled spans.
### Icons → [icons.md](./rules/icons.md)
- **Icons in `Button` use `data-icon`.** `data-icon="inline-start"` or `data-icon="inline-end"` on the icon.
- **No sizing classes on icons inside components.** Components handle icon sizing via CSS. No `size-4` or `w-4 h-4`.
- **Pass icons as objects, not string keys.** `icon={CheckIcon}`, not a string lookup.
### CLI
- **Never decode or fetch preset codes manually.** Pass them directly to `npx shadcn@latest init --preset <code>`.
## Key Patterns
These are the most common patterns that differentiate correct shadcn/ui code. For edge cases, see the linked rule files above.
```tsx
// Form layout: FieldGroup + Field, not div + Label.
<FieldGroup>
<Field>
<FieldLabel htmlFor="email">Email</FieldLabel>
<Input id="email" />
</Field>
</FieldGroup>
// Validation: data-invalid on Field, aria-invalid on the control.
<Field data-invalid>
<FieldLabel>Email</FieldLabel>
<Input aria-invalid />
<FieldDescription>Invalid email.</FieldDescription>
</Field>
// Icons in buttons: data-icon, no sizing classes.
<Button>
<SearchIcon data-icon="inline-start" />
Search
</Button>
// Spacing: gap-*, not space-y-*.
<div className="flex flex-col gap-4"> // correct
<div className="space-y-4"> // wrong
// Equal dimensions: size-*, not w-* h-*.
<Avatar className="size-10"> // correct
<Avatar className="w-10 h-10"> // wrong
// Status colors: Badge variants or semantic tokens, not raw colors.
<Badge variant="secondary">+20.1%</Badge> // correct
<span className="text-emerald-600">+20.1%</span> // wrong
```
## Component Selection
| Need | Use |
| -------------------------- | --------------------------------------------------------------------------------------------------- |
| Button/action | `Button` with appropriate variant |
| Form inputs | `Input`, `Select`, `Combobox`, `Switch`, `Checkbox`, `RadioGroup`, `Textarea`, `InputOTP`, `Slider` |
| Toggle between 2–5 options | `ToggleGroup` + `ToggleGroupItem` |
| Data display | `Table`, `Card`, `Badge`, `Avatar` |
| Navigation | `Sidebar`, `NavigationMenu`, `Breadcrumb`, `Tabs`, `Pagination` |
| Overlays | `Dialog` (modal), `Sheet` (side panel), `Drawer` (bottom sheet), `AlertDialog` (confirmation) |
| Feedback | `sonner` (toast), `Alert`, `Progress`, `Skeleton`, `Spinner` |
| Command palette | `Command` inside `Dialog` |
| Charts | `Chart` (wraps Recharts) |
| Layout | `Card`, `Separator`, `Resizable`, `ScrollArea`, `Accordion`, `Collapsible` |
| Empty states | `Empty` |
| Menus | `DropdownMenu`, `ContextMenu`, `Menubar` |
| Tooltips/info | `Tooltip`, `HoverCard`, `Popover` |
## Key Fields
The injected project context contains these key fields:
- **`aliases`** → use the actual alias prefix for imports (e.g. `@/`, `~/`), never hardcode.
- **`isRSC`** → when `true`, components using `useState`, `useEffect`, event handlers, or browser APIs need `"use client"` at the top of the file. Always reference this field when advising on the directive.
- **`tailwindVersion`** → `"v4"` uses `@theme inline` blocks; `"v3"` uses `tailwind.config.js`.
- **`tailwindCssFile`** → the global CSS file where custom CSS variables are defined. Always edit this file, never create a new one.
- **`style`** → component visual treatment (e.g. `nova`, `vega`).
- **`base`** → primitive library (`radix` or `base`). Affects component APIs and available props.
- **`iconLibrary`** → determines icon imports. Use `lucide-react` for `lucide`, `@tabler/icons-react` for `tabler`, etc. Never assume `lucide-react`.
- **`resolvedPaths`** → exact file-system destinations for components, utils, hooks, etc.
- **`framework`** → routing and file conventions (e.g. Next.js App Router vs Vite SPA).
- **`packageManager`** → use this for any non-shadcn dependency installs (e.g. `pnpm add date-fns` vs `npm install date-fns`).
See [cli.md — `info` command](./cli.md) for the full field reference.
## Component Docs, Examples, and Usage
Run `npx shadcn@latest docs <component>` to get the URLs for a component's documentation, examples, and API reference. Fetch these URLs to get the actual content.
```bash
npx shadcn@latest docs button dialog select
```
**When creating, fixing, debugging, or using a component, always run `npx shadcn@latest docs` and fetch the URLs first.** This ensures you're working with the correct API and usage patterns rather than guessing.
## Workflow
1. **Get project context** — already injected above. Run `npx shadcn@latest info` again if you need to refresh.
2. **Check installed components first** — before running `add`, always check the `components` list from project context or list the `resolvedPaths.ui` directory. Don't import components that haven't been added, and don't re-add ones already installed.
3. **Find components** — `npx shadcn@latest search`.
4. **Get docs and examples** — run `npx shadcn@latest docs <component>` to get URLs, then fetch them. Use `npx shadcn@latest view` to browse registry items you haven't installed. To preview changes to installed components, use `npx shadcn@latest add --diff`.
5. **Install or update** — `npx shadcn@latest add`. When updating existing components, use `--dry-run` and `--diff` to preview changes first (see [Updating Components](#updating-components) below).
6. **Fix imports in third-party components** — After adding components from community registries (e.g. `@bundui`, `@magicui`), check the added non-UI files for hardcoded import paths like `@/components/ui/...`. These won't match the project's actual aliases. Use `npx shadcn@latest info` to get the correct `ui` alias (e.g. `@workspace/ui/components`) and rewrite the imports accordingly. The CLI rewrites imports for its own UI files, but third-party registry components may use default paths that don't match the project.
7. **Review added components** — After adding a component or block from any registry, **always read the added files and verify they are correct**. Check for missing sub-components (e.g. `SelectItem` without `SelectGroup`), missing imports, incorrect composition, or violations of the [Critical Rules](#critical-rules). Also replace any icon imports with the project's `iconLibrary` from the project context (e.g. if the registry item uses `lucide-react` but the project uses `hugeicons`, swap the imports and icon names accordingly). Fix all issues before moving on.
8. **Registry must be explicit** — When the user asks to add a block or component, **do not guess the registry**. If no registry is specified (e.g. user says "add a login block" without specifying `@shadcn`, `@tailark`, etc.), ask which registry to use. Never default to a registry on behalf of the user.
9. **Switching presets** — Ask the user first: **reinstall**, **merge**, or **skip**?
- **Reinstall**: `npx shadcn@latest init --preset <code> --force --reinstall`. Overwrites all components.
- **Merge**: `npx shadcn@latest init --preset <code> --force --no-reinstall`, then run `npx shadcn@latest info` to list installed components, then for each installed component use `--dry-run` and `--diff` to [smart merge](#updating-components) it individually.
- **Skip**: `npx shadcn@latest init --preset <code> --force --no-reinstall`. Only updates config and CSS, leaves components as-is.
## Updating Components
When the user asks to update a component from upstream while keeping their local changes, use `--dry-run` and `--diff` to intelligently merge. **NEVER fetch raw files from GitHub manually — always use the CLI.**
1. Run `npx shadcn@latest add <component> --dry-run` to see all files that would be affected.
2. For each file, run `npx shadcn@latest add <component> --diff <file>` to see what changed upstream vs local.
3. Decide per file based on the diff:
- No local changes → safe to overwrite.
- Has local changes → read the local file, analyze the diff, and apply upstream updates while preserving local modifications.
- User says "just update everything" → use `--overwrite`, but confirm first.
4. **Never use `--overwrite` without the user's explicit approval.**
## Quick Reference
```bash
# Create a new project.
npx shadcn@latest init --name my-app --preset base-nova
npx shadcn@latest init --name my-app --preset a2r6bw --template vite
# Create a monorepo project.
npx shadcn@latest init --name my-app --preset base-nova --monorepo
npx shadcn@latest init --name my-app --preset base-nova --template next --monorepo
# Initialize existing project.
npx shadcn@latest init --preset base-nova
npx shadcn@latest init --defaults # shortcut: --template=next --preset=base-nova
# Add components.
npx shadcn@latest add button card dialog
npx shadcn@latest add @magicui/shimmer-button
npx shadcn@latest add --all
# Preview changes before adding/updating.
npx shadcn@latest add button --dry-run
npx shadcn@latest add button --diff button.tsx
npx shadcn@latest add @acme/form --view button.tsx
# Search registries.
npx shadcn@latest search @shadcn -q "sidebar"
npx shadcn@latest search @tailark -q "stats"
# Get component docs and example URLs.
npx shadcn@latest docs button dialog select
# View registry item details (for items not yet installed).
npx shadcn@latest view @shadcn/button
```
**Named presets:** `base-nova`, `radix-nova`
**Templates:** `next`, `vite`, `start`, `react-router`, `astro` (all support `--monorepo`) and `laravel` (not supported for monorepo)
**Preset codes:** Base62 strings starting with `a` (e.g. `a2r6bw`), from [ui.shadcn.com](https://ui.shadcn.com).
## Detailed References
- [rules/forms.md](./rules/forms.md) — FieldGroup, Field, InputGroup, ToggleGroup, FieldSet, validation states
- [rules/composition.md](./rules/composition.md) — Groups, overlays, Card, Tabs, Avatar, Alert, Empty, Toast, Separator, Skeleton, Badge, Button loading
- [rules/icons.md](./rules/icons.md) — data-icon, icon sizing, passing icons as objects
- [rules/styling.md](./rules/styling.md) — Semantic colors, variants, className, spacing, size, truncate, dark mode, cn(), z-index
- [rules/base-vs-radix.md](./rules/base-vs-radix.md) — asChild vs render, Select, ToggleGroup, Slider, Accordion
- [cli.md](./cli.md) — Commands, flags, presets, templates
- [customization.md](./customization.md) — Theming, CSS variables, extending components

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interface:
display_name: "shadcn/ui"
short_description: "Manages shadcn/ui components — adding, searching, fixing, debugging, styling, and composing UI."
icon_small: "./assets/shadcn-small.png"
icon_large: "./assets/shadcn.png"

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# shadcn CLI Reference
Configuration is read from `components.json`.
> **IMPORTANT:** Always run commands using the project's package runner: `npx shadcn@latest`, `pnpm dlx shadcn@latest`, or `bunx --bun shadcn@latest`. Check `packageManager` from project context to choose the right one. Examples below use `npx shadcn@latest` but substitute the correct runner for the project.
> **IMPORTANT:** Only use the flags documented below. Do not invent or guess flags — if a flag isn't listed here, it doesn't exist. The CLI auto-detects the package manager from the project's lockfile; there is no `--package-manager` flag.
## Contents
- Commands: init, add (dry-run, smart merge), search, view, docs, info, build
- Templates: next, vite, start, react-router, astro
- Presets: named, code, URL formats and fields
- Switching presets
---
## Commands
### `init` — Initialize or create a project
```bash
npx shadcn@latest init [components...] [options]
```
Initializes shadcn/ui in an existing project or creates a new project (when `--name` is provided). Optionally installs components in the same step.
| Flag | Short | Description | Default |
| ----------------------- | ----- | --------------------------------------------------------- | ------- |
| `--template <template>` | `-t` | Template (next, start, vite, next-monorepo, react-router) | — |
| `--preset [name]` | `-p` | Preset configuration (named, code, or URL) | — |
| `--yes` | `-y` | Skip confirmation prompt | `true` |
| `--defaults` | `-d` | Use defaults (`--template=next --preset=base-nova`) | `false` |
| `--force` | `-f` | Force overwrite existing configuration | `false` |
| `--cwd <cwd>` | `-c` | Working directory | current |
| `--name <name>` | `-n` | Name for new project | — |
| `--silent` | `-s` | Mute output | `false` |
| `--rtl` | | Enable RTL support | — |
| `--reinstall` | | Re-install existing UI components | `false` |
| `--monorepo` | | Scaffold a monorepo project | — |
| `--no-monorepo` | | Skip the monorepo prompt | — |
`npx shadcn@latest create` is an alias for `npx shadcn@latest init`.
### `add` — Add components
> **IMPORTANT:** To compare local components against upstream or to preview changes, ALWAYS use `npx shadcn@latest add <component> --dry-run`, `--diff`, or `--view`. NEVER fetch raw files from GitHub or other sources manually. The CLI handles registry resolution, file paths, and CSS diffing automatically.
```bash
npx shadcn@latest add [components...] [options]
```
Accepts component names, registry-prefixed names (`@magicui/shimmer-button`), URLs, or local paths.
| Flag | Short | Description | Default |
| --------------- | ----- | -------------------------------------------------------------------------------------------------------------------- | ------- |
| `--yes` | `-y` | Skip confirmation prompt | `false` |
| `--overwrite` | `-o` | Overwrite existing files | `false` |
| `--cwd <cwd>` | `-c` | Working directory | current |
| `--all` | `-a` | Add all available components | `false` |
| `--path <path>` | `-p` | Target path for the component | — |
| `--silent` | `-s` | Mute output | `false` |
| `--dry-run` | | Preview all changes without writing files | `false` |
| `--diff [path]` | | Show diffs. Without a path, shows the first 5 files. With a path, shows that file only (implies `--dry-run`) | — |
| `--view [path]` | | Show file contents. Without a path, shows the first 5 files. With a path, shows that file only (implies `--dry-run`) | — |
#### Dry-Run Mode
Use `--dry-run` to preview what `add` would do without writing any files. `--diff` and `--view` both imply `--dry-run`.
```bash
# Preview all changes.
npx shadcn@latest add button --dry-run
# Show diffs for all files (top 5).
npx shadcn@latest add button --diff
# Show the diff for a specific file.
npx shadcn@latest add button --diff button.tsx
# Show contents for all files (top 5).
npx shadcn@latest add button --view
# Show the full content of a specific file.
npx shadcn@latest add button --view button.tsx
# Works with URLs too.
npx shadcn@latest add https://api.npoint.io/abc123 --dry-run
# CSS diffs.
npx shadcn@latest add button --diff globals.css
```
**When to use dry-run:**
- When the user asks "what files will this add?" or "what will this change?" — use `--dry-run`.
- Before overwriting existing components — use `--diff` to preview the changes first.
- When the user wants to inspect component source code without installing — use `--view`.
- When checking what CSS changes would be made to `globals.css` — use `--diff globals.css`.
- When the user asks to review or audit third-party registry code before installing — use `--view` to inspect the source.
> **`npx shadcn@latest add --dry-run` vs `npx shadcn@latest view`:** Prefer `npx shadcn@latest add --dry-run/--diff/--view` over `npx shadcn@latest view` when the user wants to preview changes to their project. `npx shadcn@latest view` only shows raw registry metadata. `npx shadcn@latest add --dry-run` shows exactly what would happen in the user's project: resolved file paths, diffs against existing files, and CSS updates. Use `npx shadcn@latest view` only when the user wants to browse registry info without a project context.
#### Smart Merge from Upstream
See [Updating Components in SKILL.md](./SKILL.md#updating-components) for the full workflow.
### `search` — Search registries
```bash
npx shadcn@latest search <registries...> [options]
```
Fuzzy search across registries. Also aliased as `npx shadcn@latest list`. Without `-q`, lists all items.
| Flag | Short | Description | Default |
| ------------------- | ----- | ---------------------- | ------- |
| `--query <query>` | `-q` | Search query | — |
| `--limit <number>` | `-l` | Max items per registry | `100` |
| `--offset <number>` | `-o` | Items to skip | `0` |
| `--cwd <cwd>` | `-c` | Working directory | current |
### `view` — View item details
```bash
npx shadcn@latest view <items...> [options]
```
Displays item info including file contents. Example: `npx shadcn@latest view @shadcn/button`.
### `docs` — Get component documentation URLs
```bash
npx shadcn@latest docs <components...> [options]
```
Outputs resolved URLs for component documentation, examples, and API references. Accepts one or more component names. Fetch the URLs to get the actual content.
Example output for `npx shadcn@latest docs input button`:
```
base radix
input
docs https://ui.shadcn.com/docs/components/radix/input
examples https://raw.githubusercontent.com/.../examples/input-example.tsx
button
docs https://ui.shadcn.com/docs/components/radix/button
examples https://raw.githubusercontent.com/.../examples/button-example.tsx
```
Some components include an `api` link to the underlying library (e.g. `cmdk` for the command component).
### `diff` — Check for updates
Do not use this command. Use `npx shadcn@latest add --diff` instead.
### `info` — Project information
```bash
npx shadcn@latest info [options]
```
Displays project info and `components.json` configuration. Run this first to discover the project's framework, aliases, Tailwind version, and resolved paths.
| Flag | Short | Description | Default |
| ------------- | ----- | ----------------- | ------- |
| `--cwd <cwd>` | `-c` | Working directory | current |
**Project Info fields:**
| Field | Type | Meaning |
| -------------------- | --------- | ------------------------------------------------------------------ |
| `framework` | `string` | Detected framework (`next`, `vite`, `react-router`, `start`, etc.) |
| `frameworkVersion` | `string` | Framework version (e.g. `15.2.4`) |
| `isSrcDir` | `boolean` | Whether the project uses a `src/` directory |
| `isRSC` | `boolean` | Whether React Server Components are enabled |
| `isTsx` | `boolean` | Whether the project uses TypeScript |
| `tailwindVersion` | `string` | `"v3"` or `"v4"` |
| `tailwindConfigFile` | `string` | Path to the Tailwind config file |
| `tailwindCssFile` | `string` | Path to the global CSS file |
| `aliasPrefix` | `string` | Import alias prefix (e.g. `@`, `~`, `@/`) |
| `packageManager` | `string` | Detected package manager (`npm`, `pnpm`, `yarn`, `bun`) |
**Components.json fields:**
| Field | Type | Meaning |
| -------------------- | --------- | ------------------------------------------------------------------------------------------ |
| `base` | `string` | Primitive library (`radix` or `base`) — determines component APIs and available props |
| `style` | `string` | Visual style (e.g. `nova`, `vega`) |
| `rsc` | `boolean` | RSC flag from config |
| `tsx` | `boolean` | TypeScript flag |
| `tailwind.config` | `string` | Tailwind config path |
| `tailwind.css` | `string` | Global CSS path — this is where custom CSS variables go |
| `iconLibrary` | `string` | Icon library — determines icon import package (e.g. `lucide-react`, `@tabler/icons-react`) |
| `aliases.components` | `string` | Component import alias (e.g. `@/components`) |
| `aliases.utils` | `string` | Utils import alias (e.g. `@/lib/utils`) |
| `aliases.ui` | `string` | UI component alias (e.g. `@/components/ui`) |
| `aliases.lib` | `string` | Lib alias (e.g. `@/lib`) |
| `aliases.hooks` | `string` | Hooks alias (e.g. `@/hooks`) |
| `resolvedPaths` | `object` | Absolute file-system paths for each alias |
| `registries` | `object` | Configured custom registries |
**Links fields:**
The `info` output includes a **Links** section with templated URLs for component docs, source, and examples. For resolved URLs, use `npx shadcn@latest docs <component>` instead.
### `build` — Build a custom registry
```bash
npx shadcn@latest build [registry] [options]
```
Builds `registry.json` into individual JSON files for distribution. Default input: `./registry.json`, default output: `./public/r`.
| Flag | Short | Description | Default |
| ----------------- | ----- | ----------------- | ------------ |
| `--output <path>` | `-o` | Output directory | `./public/r` |
| `--cwd <cwd>` | `-c` | Working directory | current |
---
## Templates
| Value | Framework | Monorepo support |
| -------------- | -------------- | ---------------- |
| `next` | Next.js | Yes |
| `vite` | Vite | Yes |
| `start` | TanStack Start | Yes |
| `react-router` | React Router | Yes |
| `astro` | Astro | Yes |
| `laravel` | Laravel | No |
All templates support monorepo scaffolding via the `--monorepo` flag. When passed, the CLI uses a monorepo-specific template directory (e.g. `next-monorepo`, `vite-monorepo`). When neither `--monorepo` nor `--no-monorepo` is passed, the CLI prompts interactively. Laravel does not support monorepo scaffolding.
---
## Presets
Three ways to specify a preset via `--preset`:
1. **Named:** `--preset base-nova` or `--preset radix-nova`
2. **Code:** `--preset a2r6bw` (base62 string, starts with lowercase `a`)
3. **URL:** `--preset "https://ui.shadcn.com/init?base=radix&style=nova&..."`
> **IMPORTANT:** Never try to decode, fetch, or resolve preset codes manually. Preset codes are opaque — pass them directly to `npx shadcn@latest init --preset <code>` and let the CLI handle resolution.
## Switching Presets
Ask the user first: **reinstall**, **merge**, or **skip** existing components?
- **Re-install** → `npx shadcn@latest init --preset <code> --force --reinstall`. Overwrites all component files with the new preset styles. Use when the user hasn't customized components.
- **Merge** → `npx shadcn@latest init --preset <code> --force --no-reinstall`, then run `npx shadcn@latest info` to get the list of installed components and use the [smart merge workflow](./SKILL.md#updating-components) to update them one by one, preserving local changes. Use when the user has customized components.
- **Skip** → `npx shadcn@latest init --preset <code> --force --no-reinstall`. Only updates config and CSS variables, leaves existing components as-is.

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# Customization & Theming
Components reference semantic CSS variable tokens. Change the variables to change every component.
## Contents
- How it works (CSS variables → Tailwind utilities → components)
- Color variables and OKLCH format
- Dark mode setup
- Changing the theme (presets, CSS variables)
- Adding custom colors (Tailwind v3 and v4)
- Border radius
- Customizing components (variants, className, wrappers)
- Checking for updates
---
## How It Works
1. CSS variables defined in `:root` (light) and `.dark` (dark mode).
2. Tailwind maps them to utilities: `bg-primary`, `text-muted-foreground`, etc.
3. Components use these utilities — changing a variable changes all components that reference it.
---
## Color Variables
Every color follows the `name` / `name-foreground` convention. The base variable is for backgrounds, `-foreground` is for text/icons on that background.
| Variable | Purpose |
| -------------------------------------------- | -------------------------------- |
| `--background` / `--foreground` | Page background and default text |
| `--card` / `--card-foreground` | Card surfaces |
| `--primary` / `--primary-foreground` | Primary buttons and actions |
| `--secondary` / `--secondary-foreground` | Secondary actions |
| `--muted` / `--muted-foreground` | Muted/disabled states |
| `--accent` / `--accent-foreground` | Hover and accent states |
| `--destructive` / `--destructive-foreground` | Error and destructive actions |
| `--border` | Default border color |
| `--input` | Form input borders |
| `--ring` | Focus ring color |
| `--chart-1` through `--chart-5` | Chart/data visualization |
| `--sidebar-*` | Sidebar-specific colors |
| `--surface` / `--surface-foreground` | Secondary surface |
Colors use OKLCH: `--primary: oklch(0.205 0 0)` where values are lightness (0–1), chroma (0 = gray), and hue (0–360).
---
## Dark Mode
Class-based toggle via `.dark` on the root element. In Next.js, use `next-themes`:
```tsx
import { ThemeProvider } from "next-themes"
<ThemeProvider attribute="class" defaultTheme="system" enableSystem>
{children}
</ThemeProvider>
```
---
## Changing the Theme
```bash
# Apply a preset code from ui.shadcn.com.
npx shadcn@latest init --preset a2r6bw --force
# Switch to a named preset.
npx shadcn@latest init --preset radix-nova --force
npx shadcn@latest init --reinstall # update existing components to match
# Use a custom theme URL.
npx shadcn@latest init --preset "https://ui.shadcn.com/init?base=radix&style=nova&theme=blue&..." --force
```
Or edit CSS variables directly in `globals.css`.
---
## Adding Custom Colors
Add variables to the file at `tailwindCssFile` from `npx shadcn@latest info` (typically `globals.css`). Never create a new CSS file for this.
```css
/* 1. Define in the global CSS file. */
:root {
--warning: oklch(0.84 0.16 84);
--warning-foreground: oklch(0.28 0.07 46);
}
.dark {
--warning: oklch(0.41 0.11 46);
--warning-foreground: oklch(0.99 0.02 95);
}
```
```css
/* 2a. Register with Tailwind v4 (@theme inline). */
@theme inline {
--color-warning: var(--warning);
--color-warning-foreground: var(--warning-foreground);
}
```
When `tailwindVersion` is `"v3"` (check via `npx shadcn@latest info`), register in `tailwind.config.js` instead:
```js
// 2b. Register with Tailwind v3 (tailwind.config.js).
module.exports = {
theme: {
extend: {
colors: {
warning: "oklch(var(--warning) / <alpha-value>)",
"warning-foreground":
"oklch(var(--warning-foreground) / <alpha-value>)",
},
},
},
}
```
```tsx
// 3. Use in components.
<div className="bg-warning text-warning-foreground">Warning</div>
```
---
## Border Radius
`--radius` controls border radius globally. Components derive values from it (`rounded-lg` = `var(--radius)`, `rounded-md` = `calc(var(--radius) - 2px)`).
---
## Customizing Components
See also: [rules/styling.md](./rules/styling.md) for Incorrect/Correct examples.
Prefer these approaches in order:
### 1. Built-in variants
```tsx
<Button variant="outline" size="sm">Click</Button>
```
### 2. Tailwind classes via `className`
```tsx
<Card className="max-w-md mx-auto">...</Card>
```
### 3. Add a new variant
Edit the component source to add a variant via `cva`:
```tsx
// components/ui/button.tsx
warning: "bg-warning text-warning-foreground hover:bg-warning/90",
```
### 4. Wrapper components
Compose shadcn/ui primitives into higher-level components:
```tsx
export function ConfirmDialog({ title, description, onConfirm, children }) {
return (
<AlertDialog>
<AlertDialogTrigger asChild>{children}</AlertDialogTrigger>
<AlertDialogContent>
<AlertDialogHeader>
<AlertDialogTitle>{title}</AlertDialogTitle>
<AlertDialogDescription>{description}</AlertDialogDescription>
</AlertDialogHeader>
<AlertDialogFooter>
<AlertDialogCancel>Cancel</AlertDialogCancel>
<AlertDialogAction onClick={onConfirm}>Confirm</AlertDialogAction>
</AlertDialogFooter>
</AlertDialogContent>
</AlertDialog>
)
}
```
---
## Checking for Updates
```bash
npx shadcn@latest add button --diff
```
To preview exactly what would change before updating, use `--dry-run` and `--diff`:
```bash
npx shadcn@latest add button --dry-run # see all affected files
npx shadcn@latest add button --diff button.tsx # see the diff for a specific file
```
See [Updating Components in SKILL.md](./SKILL.md#updating-components) for the full smart merge workflow.

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{
"skill_name": "shadcn",
"evals": [
{
"id": 1,
"prompt": "I'm building a Next.js app with shadcn/ui (base-nova preset, lucide icons). Create a settings form component with fields for: full name, email address, and notification preferences (email, SMS, push notifications as toggle options). Add validation states for required fields.",
"expected_output": "A React component using FieldGroup, Field, ToggleGroup, data-invalid/aria-invalid validation, gap-* spacing, and semantic colors.",
"files": [],
"expectations": [
"Uses FieldGroup and Field components for form layout instead of raw div with space-y",
"Uses Switch for independent on/off notification toggles (not looping Button with manual active state)",
"Uses data-invalid on Field and aria-invalid on the input control for validation states",
"Uses gap-* (e.g. gap-4, gap-6) instead of space-y-* or space-x-* for spacing",
"Uses semantic color tokens (e.g. bg-background, text-muted-foreground, text-destructive) instead of raw colors like bg-red-500",
"No manual dark: color overrides"
]
},
{
"id": 2,
"prompt": "Create a dialog component for editing a user profile. It should have the user's avatar at the top, input fields for name and bio, and Save/Cancel buttons with appropriate icons. Using shadcn/ui with radix-nova preset and tabler icons.",
"expected_output": "A React component with DialogTitle, Avatar+AvatarFallback, data-icon on icon buttons, no icon sizing classes, tabler icon imports.",
"files": [],
"expectations": [
"Includes DialogTitle for accessibility (visible or with sr-only class)",
"Avatar component includes AvatarFallback",
"Icons on buttons use the data-icon attribute (data-icon=\"inline-start\" or data-icon=\"inline-end\")",
"No sizing classes on icons inside components (no size-4, w-4, h-4, etc.)",
"Uses tabler icons (@tabler/icons-react) instead of lucide-react",
"Uses asChild for custom triggers (radix preset)"
]
},
{
"id": 3,
"prompt": "Create a dashboard component that shows 4 stat cards in a grid. Each card has a title, large number, percentage change badge, and a loading skeleton state. Using shadcn/ui with base-nova preset and lucide icons.",
"expected_output": "A React component with full Card composition, Skeleton for loading, Badge for changes, semantic colors, gap-* spacing.",
"files": [],
"expectations": [
"Uses full Card composition with CardHeader, CardTitle, CardContent (not dumping everything into CardContent)",
"Uses Skeleton component for loading placeholders instead of custom animate-pulse divs",
"Uses Badge component for percentage change instead of custom styled spans",
"Uses semantic color tokens instead of raw color values like bg-green-500 or text-red-600",
"Uses gap-* instead of space-y-* or space-x-* for spacing",
"Uses size-* when width and height are equal instead of separate w-* h-*"
]
}
]
}

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# shadcn MCP Server
The CLI includes an MCP server that lets AI assistants search, browse, view, and install components from registries.
---
## Setup
```bash
shadcn mcp # start the MCP server (stdio)
shadcn mcp init # write config for your editor
```
Editor config files:
| Editor | Config file |
|--------|------------|
| Claude Code | `.mcp.json` |
| Cursor | `.cursor/mcp.json` |
| VS Code | `.vscode/mcp.json` |
| OpenCode | `opencode.json` |
| Codex | `~/.codex/config.toml` (manual) |
---
## Tools
> **Tip:** MCP tools handle registry operations (search, view, install). For project configuration (aliases, framework, Tailwind version), use `npx shadcn@latest info` — there is no MCP equivalent.
### `shadcn:get_project_registries`
Returns registry names from `components.json`. Errors if no `components.json` exists.
**Input:** none
### `shadcn:list_items_in_registries`
Lists all items from one or more registries.
**Input:** `registries` (string[]), `limit` (number, optional), `offset` (number, optional)
### `shadcn:search_items_in_registries`
Fuzzy search across registries.
**Input:** `registries` (string[]), `query` (string), `limit` (number, optional), `offset` (number, optional)
### `shadcn:view_items_in_registries`
View item details including full file contents.
**Input:** `items` (string[]) — e.g. `["@shadcn/button", "@shadcn/card"]`
### `shadcn:get_item_examples_from_registries`
Find usage examples and demos with source code.
**Input:** `registries` (string[]), `query` (string) — e.g. `"accordion-demo"`, `"button example"`
### `shadcn:get_add_command_for_items`
Returns the CLI install command.
**Input:** `items` (string[]) — e.g. `["@shadcn/button"]`
### `shadcn:get_audit_checklist`
Returns a checklist for verifying components (imports, deps, lint, TypeScript).
**Input:** none
---
## Configuring Registries
Registries are set in `components.json`. The `@shadcn` registry is always built-in.
```json
{
"registries": {
"@acme": "https://acme.com/r/{name}.json",
"@private": {
"url": "https://private.com/r/{name}.json",
"headers": { "Authorization": "Bearer ${MY_TOKEN}" }
}
}
}
```
- Names must start with `@`.
- URLs must contain `{name}`.
- `${VAR}` references are resolved from environment variables.
Community registry index: `https://ui.shadcn.com/r/registries.json`

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# Base vs Radix
API differences between `base` and `radix`. Check the `base` field from `npx shadcn@latest info`.
## Contents
- Composition: asChild vs render
- Button / trigger as non-button element
- Select (items prop, placeholder, positioning, multiple, object values)
- ToggleGroup (type vs multiple)
- Slider (scalar vs array)
- Accordion (type and defaultValue)
---
## Composition: asChild (radix) vs render (base)
Radix uses `asChild` to replace the default element. Base uses `render`. Don't wrap triggers in extra elements.
**Incorrect:**
```tsx
<DialogTrigger>
<div>
<Button>Open</Button>
</div>
</DialogTrigger>
```
**Correct (radix):**
```tsx
<DialogTrigger asChild>
<Button>Open</Button>
</DialogTrigger>
```
**Correct (base):**
```tsx
<DialogTrigger render={<Button />}>Open</DialogTrigger>
```
This applies to all trigger and close components: `DialogTrigger`, `SheetTrigger`, `AlertDialogTrigger`, `DropdownMenuTrigger`, `PopoverTrigger`, `TooltipTrigger`, `CollapsibleTrigger`, `DialogClose`, `SheetClose`, `NavigationMenuLink`, `BreadcrumbLink`, `SidebarMenuButton`, `Badge`, `Item`.
---
## Button / trigger as non-button element (base only)
When `render` changes an element to a non-button (`<a>`, `<span>`), add `nativeButton={false}`.
**Incorrect (base):** missing `nativeButton={false}`.
```tsx
<Button render={<a href="/docs" />}>Read the docs</Button>
```
**Correct (base):**
```tsx
<Button render={<a href="/docs" />} nativeButton={false}>
Read the docs
</Button>
```
**Correct (radix):**
```tsx
<Button asChild>
<a href="/docs">Read the docs</a>
</Button>
```
Same for triggers whose `render` is not a `Button`:
```tsx
// base.
<PopoverTrigger render={<InputGroupAddon />} nativeButton={false}>
Pick date
</PopoverTrigger>
```
---
## Select
**items prop (base only).** Base requires an `items` prop on the root. Radix uses inline JSX only.
**Incorrect (base):**
```tsx
<Select>
<SelectTrigger><SelectValue placeholder="Select a fruit" /></SelectTrigger>
</Select>
```
**Correct (base):**
```tsx
const items = [
{ label: "Select a fruit", value: null },
{ label: "Apple", value: "apple" },
{ label: "Banana", value: "banana" },
]
<Select items={items}>
<SelectTrigger>
<SelectValue />
</SelectTrigger>
<SelectContent>
<SelectGroup>
{items.map((item) => (
<SelectItem key={item.value} value={item.value}>{item.label}</SelectItem>
))}
</SelectGroup>
</SelectContent>
</Select>
```
**Correct (radix):**
```tsx
<Select>
<SelectTrigger>
<SelectValue placeholder="Select a fruit" />
</SelectTrigger>
<SelectContent>
<SelectGroup>
<SelectItem value="apple">Apple</SelectItem>
<SelectItem value="banana">Banana</SelectItem>
</SelectGroup>
</SelectContent>
</Select>
```
**Placeholder.** Base uses a `{ value: null }` item in the items array. Radix uses `<SelectValue placeholder="...">`.
**Content positioning.** Base uses `alignItemWithTrigger`. Radix uses `position`.
```tsx
// base.
<SelectContent alignItemWithTrigger={false} side="bottom">
// radix.
<SelectContent position="popper">
```
---
## Select — multiple selection and object values (base only)
Base supports `multiple`, render-function children on `SelectValue`, and object values with `itemToStringValue`. Radix is single-select with string values only.
**Correct (base — multiple selection):**
```tsx
<Select items={items} multiple defaultValue={[]}>
<SelectTrigger>
<SelectValue>
{(value: string[]) => value.length === 0 ? "Select fruits" : `${value.length} selected`}
</SelectValue>
</SelectTrigger>
...
</Select>
```
**Correct (base — object values):**
```tsx
<Select defaultValue={plans[0]} itemToStringValue={(plan) => plan.name}>
<SelectTrigger>
<SelectValue>{(value) => value.name}</SelectValue>
</SelectTrigger>
...
</Select>
```
---
## ToggleGroup
Base uses a `multiple` boolean prop. Radix uses `type="single"` or `type="multiple"`.
**Incorrect (base):**
```tsx
<ToggleGroup type="single" defaultValue="daily">
<ToggleGroupItem value="daily">Daily</ToggleGroupItem>
</ToggleGroup>
```
**Correct (base):**
```tsx
// Single (no prop needed), defaultValue is always an array.
<ToggleGroup defaultValue={["daily"]} spacing={2}>
<ToggleGroupItem value="daily">Daily</ToggleGroupItem>
<ToggleGroupItem value="weekly">Weekly</ToggleGroupItem>
</ToggleGroup>
// Multi-selection.
<ToggleGroup multiple>
<ToggleGroupItem value="bold">Bold</ToggleGroupItem>
<ToggleGroupItem value="italic">Italic</ToggleGroupItem>
</ToggleGroup>
```
**Correct (radix):**
```tsx
// Single, defaultValue is a string.
<ToggleGroup type="single" defaultValue="daily" spacing={2}>
<ToggleGroupItem value="daily">Daily</ToggleGroupItem>
<ToggleGroupItem value="weekly">Weekly</ToggleGroupItem>
</ToggleGroup>
// Multi-selection.
<ToggleGroup type="multiple">
<ToggleGroupItem value="bold">Bold</ToggleGroupItem>
<ToggleGroupItem value="italic">Italic</ToggleGroupItem>
</ToggleGroup>
```
**Controlled single value:**
```tsx
// base — wrap/unwrap arrays.
const [value, setValue] = React.useState("normal")
<ToggleGroup value={[value]} onValueChange={(v) => setValue(v[0])}>
// radix — plain string.
const [value, setValue] = React.useState("normal")
<ToggleGroup type="single" value={value} onValueChange={setValue}>
```
---
## Slider
Base accepts a plain number for a single thumb. Radix always requires an array.
**Incorrect (base):**
```tsx
<Slider defaultValue={[50]} max={100} step={1} />
```
**Correct (base):**
```tsx
<Slider defaultValue={50} max={100} step={1} />
```
**Correct (radix):**
```tsx
<Slider defaultValue={[50]} max={100} step={1} />
```
Both use arrays for range sliders. Controlled `onValueChange` in base may need a cast:
```tsx
// base.
const [value, setValue] = React.useState([0.3, 0.7])
<Slider value={value} onValueChange={(v) => setValue(v as number[])} />
// radix.
const [value, setValue] = React.useState([0.3, 0.7])
<Slider value={value} onValueChange={setValue} />
```
---
## Accordion
Radix requires `type="single"` or `type="multiple"` and supports `collapsible`. `defaultValue` is a string. Base uses no `type` prop, uses `multiple` boolean, and `defaultValue` is always an array.
**Incorrect (base):**
```tsx
<Accordion type="single" collapsible defaultValue="item-1">
<AccordionItem value="item-1">...</AccordionItem>
</Accordion>
```
**Correct (base):**
```tsx
<Accordion defaultValue={["item-1"]}>
<AccordionItem value="item-1">...</AccordionItem>
</Accordion>
// Multi-select.
<Accordion multiple defaultValue={["item-1", "item-2"]}>
<AccordionItem value="item-1">...</AccordionItem>
<AccordionItem value="item-2">...</AccordionItem>
</Accordion>
```
**Correct (radix):**
```tsx
<Accordion type="single" collapsible defaultValue="item-1">
<AccordionItem value="item-1">...</AccordionItem>
</Accordion>
```

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# Component Composition
## Contents
- Items always inside their Group component
- Callouts use Alert
- Empty states use Empty component
- Toast notifications use sonner
- Choosing between overlay components
- Dialog, Sheet, and Drawer always need a Title
- Card structure
- Button has no isPending or isLoading prop
- TabsTrigger must be inside TabsList
- Avatar always needs AvatarFallback
- Use Separator instead of raw hr or border divs
- Use Skeleton for loading placeholders
- Use Badge instead of custom styled spans
---
## Items always inside their Group component
Never render items directly inside the content container.
**Incorrect:**
```tsx
<SelectContent>
<SelectItem value="apple">Apple</SelectItem>
<SelectItem value="banana">Banana</SelectItem>
</SelectContent>
```
**Correct:**
```tsx
<SelectContent>
<SelectGroup>
<SelectItem value="apple">Apple</SelectItem>
<SelectItem value="banana">Banana</SelectItem>
</SelectGroup>
</SelectContent>
```
This applies to all group-based components:
| Item | Group |
|------|-------|
| `SelectItem`, `SelectLabel` | `SelectGroup` |
| `DropdownMenuItem`, `DropdownMenuLabel`, `DropdownMenuSub` | `DropdownMenuGroup` |
| `MenubarItem` | `MenubarGroup` |
| `ContextMenuItem` | `ContextMenuGroup` |
| `CommandItem` | `CommandGroup` |
---
## Callouts use Alert
```tsx
<Alert>
<AlertTitle>Warning</AlertTitle>
<AlertDescription>Something needs attention.</AlertDescription>
</Alert>
```
---
## Empty states use Empty component
```tsx
<Empty>
<EmptyHeader>
<EmptyMedia variant="icon"><FolderIcon /></EmptyMedia>
<EmptyTitle>No projects yet</EmptyTitle>
<EmptyDescription>Get started by creating a new project.</EmptyDescription>
</EmptyHeader>
<EmptyContent>
<Button>Create Project</Button>
</EmptyContent>
</Empty>
```
---
## Toast notifications use sonner
```tsx
import { toast } from "sonner"
toast.success("Changes saved.")
toast.error("Something went wrong.")
toast("File deleted.", {
action: { label: "Undo", onClick: () => undoDelete() },
})
```
---
## Choosing between overlay components
| Use case | Component |
|----------|-----------|
| Focused task that requires input | `Dialog` |
| Destructive action confirmation | `AlertDialog` |
| Side panel with details or filters | `Sheet` |
| Mobile-first bottom panel | `Drawer` |
| Quick info on hover | `HoverCard` |
| Small contextual content on click | `Popover` |
---
## Dialog, Sheet, and Drawer always need a Title
`DialogTitle`, `SheetTitle`, `DrawerTitle` are required for accessibility. Use `className="sr-only"` if visually hidden.
```tsx
<DialogContent>
<DialogHeader>
<DialogTitle>Edit Profile</DialogTitle>
<DialogDescription>Update your profile.</DialogDescription>
</DialogHeader>
...
</DialogContent>
```
---
## Card structure
Use full composition — don't dump everything into `CardContent`:
```tsx
<Card>
<CardHeader>
<CardTitle>Team Members</CardTitle>
<CardDescription>Manage your team.</CardDescription>
</CardHeader>
<CardContent>...</CardContent>
<CardFooter>
<Button>Invite</Button>
</CardFooter>
</Card>
```
---
## Button has no isPending or isLoading prop
Compose with `Spinner` + `data-icon` + `disabled`:
```tsx
<Button disabled>
<Spinner data-icon="inline-start" />
Saving...
</Button>
```
---
## TabsTrigger must be inside TabsList
Never render `TabsTrigger` directly inside `Tabs` — always wrap in `TabsList`:
```tsx
<Tabs defaultValue="account">
<TabsList>
<TabsTrigger value="account">Account</TabsTrigger>
<TabsTrigger value="password">Password</TabsTrigger>
</TabsList>
<TabsContent value="account">...</TabsContent>
</Tabs>
```
---
## Avatar always needs AvatarFallback
Always include `AvatarFallback` for when the image fails to load:
```tsx
<Avatar>
<AvatarImage src="/avatar.png" alt="User" />
<AvatarFallback>JD</AvatarFallback>
</Avatar>
```
---
## Use existing components instead of custom markup
| Instead of | Use |
|---|---|
| `<hr>` or `<div className="border-t">` | `<Separator />` |
| `<div className="animate-pulse">` with styled divs | `<Skeleton className="h-4 w-3/4" />` |
| `<span className="rounded-full bg-green-100 ...">` | `<Badge variant="secondary">` |

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# Forms & Inputs
## Contents
- Forms use FieldGroup + Field
- InputGroup requires InputGroupInput/InputGroupTextarea
- Buttons inside inputs use InputGroup + InputGroupAddon
- Option sets (2–7 choices) use ToggleGroup
- FieldSet + FieldLegend for grouping related fields
- Field validation and disabled states
---
## Forms use FieldGroup + Field
Always use `FieldGroup` + `Field` — never raw `div` with `space-y-*`:
```tsx
<FieldGroup>
<Field>
<FieldLabel htmlFor="email">Email</FieldLabel>
<Input id="email" type="email" />
</Field>
<Field>
<FieldLabel htmlFor="password">Password</FieldLabel>
<Input id="password" type="password" />
</Field>
</FieldGroup>
```
Use `Field orientation="horizontal"` for settings pages. Use `FieldLabel className="sr-only"` for visually hidden labels.
**Choosing form controls:**
- Simple text input → `Input`
- Dropdown with predefined options → `Select`
- Searchable dropdown → `Combobox`
- Native HTML select (no JS) → `native-select`
- Boolean toggle → `Switch` (for settings) or `Checkbox` (for forms)
- Single choice from few options → `RadioGroup`
- Toggle between 2–5 options → `ToggleGroup` + `ToggleGroupItem`
- OTP/verification code → `InputOTP`
- Multi-line text → `Textarea`
---
## InputGroup requires InputGroupInput/InputGroupTextarea
Never use raw `Input` or `Textarea` inside an `InputGroup`.
**Incorrect:**
```tsx
<InputGroup>
<Input placeholder="Search..." />
</InputGroup>
```
**Correct:**
```tsx
import { InputGroup, InputGroupInput } from "@/components/ui/input-group"
<InputGroup>
<InputGroupInput placeholder="Search..." />
</InputGroup>
```
---
## Buttons inside inputs use InputGroup + InputGroupAddon
Never place a `Button` directly inside or adjacent to an `Input` with custom positioning.
**Incorrect:**
```tsx
<div className="relative">
<Input placeholder="Search..." className="pr-10" />
<Button className="absolute right-0 top-0" size="icon">
<SearchIcon />
</Button>
</div>
```
**Correct:**
```tsx
import { InputGroup, InputGroupInput, InputGroupAddon } from "@/components/ui/input-group"
<InputGroup>
<InputGroupInput placeholder="Search..." />
<InputGroupAddon>
<Button size="icon">
<SearchIcon data-icon="inline-start" />
</Button>
</InputGroupAddon>
</InputGroup>
```
---
## Option sets (2–7 choices) use ToggleGroup
Don't manually loop `Button` components with active state.
**Incorrect:**
```tsx
const [selected, setSelected] = useState("daily")
<div className="flex gap-2">
{["daily", "weekly", "monthly"].map((option) => (
<Button
key={option}
variant={selected === option ? "default" : "outline"}
onClick={() => setSelected(option)}
>
{option}
</Button>
))}
</div>
```
**Correct:**
```tsx
import { ToggleGroup, ToggleGroupItem } from "@/components/ui/toggle-group"
<ToggleGroup spacing={2}>
<ToggleGroupItem value="daily">Daily</ToggleGroupItem>
<ToggleGroupItem value="weekly">Weekly</ToggleGroupItem>
<ToggleGroupItem value="monthly">Monthly</ToggleGroupItem>
</ToggleGroup>
```
Combine with `Field` for labelled toggle groups:
```tsx
<Field orientation="horizontal">
<FieldTitle id="theme-label">Theme</FieldTitle>
<ToggleGroup aria-labelledby="theme-label" spacing={2}>
<ToggleGroupItem value="light">Light</ToggleGroupItem>
<ToggleGroupItem value="dark">Dark</ToggleGroupItem>
<ToggleGroupItem value="system">System</ToggleGroupItem>
</ToggleGroup>
</Field>
```
> **Note:** `defaultValue` and `type`/`multiple` props differ between base and radix. See [base-vs-radix.md](./base-vs-radix.md#togglegroup).
---
## FieldSet + FieldLegend for grouping related fields
Use `FieldSet` + `FieldLegend` for related checkboxes, radios, or switches — not `div` with a heading:
```tsx
<FieldSet>
<FieldLegend variant="label">Preferences</FieldLegend>
<FieldDescription>Select all that apply.</FieldDescription>
<FieldGroup className="gap-3">
<Field orientation="horizontal">
<Checkbox id="dark" />
<FieldLabel htmlFor="dark" className="font-normal">Dark mode</FieldLabel>
</Field>
</FieldGroup>
</FieldSet>
```
---
## Field validation and disabled states
Both attributes are needed — `data-invalid`/`data-disabled` styles the field (label, description), while `aria-invalid`/`disabled` styles the control.
```tsx
// Invalid.
<Field data-invalid>
<FieldLabel htmlFor="email">Email</FieldLabel>
<Input id="email" aria-invalid />
<FieldDescription>Invalid email address.</FieldDescription>
</Field>
// Disabled.
<Field data-disabled>
<FieldLabel htmlFor="email">Email</FieldLabel>
<Input id="email" disabled />
</Field>
```
Works for all controls: `Input`, `Textarea`, `Select`, `Checkbox`, `RadioGroupItem`, `Switch`, `Slider`, `NativeSelect`, `InputOTP`.

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# Icons
**Always use the project's configured `iconLibrary` for imports.** Check the `iconLibrary` field from project context: `lucide` → `lucide-react`, `tabler` → `@tabler/icons-react`, etc. Never assume `lucide-react`.
---
## Icons in Button use data-icon attribute
Add `data-icon="inline-start"` (prefix) or `data-icon="inline-end"` (suffix) to the icon. No sizing classes on the icon.
**Incorrect:**
```tsx
<Button>
<SearchIcon className="mr-2 size-4" />
Search
</Button>
```
**Correct:**
```tsx
<Button>
<SearchIcon data-icon="inline-start"/>
Search
</Button>
<Button>
Next
<ArrowRightIcon data-icon="inline-end"/>
</Button>
```
---
## No sizing classes on icons inside components
Components handle icon sizing via CSS. Don't add `size-4`, `w-4 h-4`, or other sizing classes to icons inside `Button`, `DropdownMenuItem`, `Alert`, `Sidebar*`, or other shadcn components. Unless the user explicitly asks for custom icon sizes.
**Incorrect:**
```tsx
<Button>
<SearchIcon className="size-4" data-icon="inline-start" />
Search
</Button>
<DropdownMenuItem>
<SettingsIcon className="mr-2 size-4" />
Settings
</DropdownMenuItem>
```
**Correct:**
```tsx
<Button>
<SearchIcon data-icon="inline-start" />
Search
</Button>
<DropdownMenuItem>
<SettingsIcon />
Settings
</DropdownMenuItem>
```
---
## Pass icons as component objects, not string keys
Use `icon={CheckIcon}`, not a string key to a lookup map.
**Incorrect:**
```tsx
const iconMap = {
check: CheckIcon,
alert: AlertIcon,
}
function StatusBadge({ icon }: { icon: string }) {
const Icon = iconMap[icon]
return <Icon />
}
<StatusBadge icon="check" />
```
**Correct:**
```tsx
// Import from the project's configured iconLibrary (e.g. lucide-react, @tabler/icons-react).
import { CheckIcon } from "lucide-react"
function StatusBadge({ icon: Icon }: { icon: React.ComponentType }) {
return <Icon />
}
<StatusBadge icon={CheckIcon} />
```

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# Styling & Customization
See [customization.md](../customization.md) for theming, CSS variables, and adding custom colors.
## Contents
- Semantic colors
- Built-in variants first
- className for layout only
- No space-x-* / space-y-*
- Prefer size-* over w-* h-* when equal
- Prefer truncate shorthand
- No manual dark: color overrides
- Use cn() for conditional classes
- No manual z-index on overlay components
---
## Semantic colors
**Incorrect:**
```tsx
<div className="bg-blue-500 text-white">
<p className="text-gray-600">Secondary text</p>
</div>
```
**Correct:**
```tsx
<div className="bg-primary text-primary-foreground">
<p className="text-muted-foreground">Secondary text</p>
</div>
```
---
## No raw color values for status/state indicators
For positive, negative, or status indicators, use Badge variants, semantic tokens like `text-destructive`, or define custom CSS variables — don't reach for raw Tailwind colors.
**Incorrect:**
```tsx
<span className="text-emerald-600">+20.1%</span>
<span className="text-green-500">Active</span>
<span className="text-red-600">-3.2%</span>
```
**Correct:**
```tsx
<Badge variant="secondary">+20.1%</Badge>
<Badge>Active</Badge>
<span className="text-destructive">-3.2%</span>
```
If you need a success/positive color that doesn't exist as a semantic token, use a Badge variant or ask the user about adding a custom CSS variable to the theme (see [customization.md](../customization.md)).
---
## Built-in variants first
**Incorrect:**
```tsx
<Button className="border border-input bg-transparent hover:bg-accent">
Click me
</Button>
```
**Correct:**
```tsx
<Button variant="outline">Click me</Button>
```
---
## className for layout only
Use `className` for layout (e.g. `max-w-md`, `mx-auto`, `mt-4`), **not** for overriding component colors or typography. To change colors, use semantic tokens, built-in variants, or CSS variables.
**Incorrect:**
```tsx
<Card className="bg-blue-100 text-blue-900 font-bold">
<CardContent>Dashboard</CardContent>
</Card>
```
**Correct:**
```tsx
<Card className="max-w-md mx-auto">
<CardContent>Dashboard</CardContent>
</Card>
```
To customize a component's appearance, prefer these approaches in order:
1. **Built-in variants** — `variant="outline"`, `variant="destructive"`, etc.
2. **Semantic color tokens** — `bg-primary`, `text-muted-foreground`.
3. **CSS variables** — define custom colors in the global CSS file (see [customization.md](../customization.md)).
---
## No space-x-* / space-y-*
Use `gap-*` instead. `space-y-4` → `flex flex-col gap-4`. `space-x-2` → `flex gap-2`.
```tsx
<div className="flex flex-col gap-4">
<Input />
<Input />
<Button>Submit</Button>
</div>
```
---
## Prefer size-* over w-* h-* when equal
`size-10` not `w-10 h-10`. Applies to icons, avatars, skeletons, etc.
---
## Prefer truncate shorthand
`truncate` not `overflow-hidden text-ellipsis whitespace-nowrap`.
---
## No manual dark: color overrides
Use semantic tokens — they handle light/dark via CSS variables. `bg-background text-foreground` not `bg-white dark:bg-gray-950`.
---
## Use cn() for conditional classes
Use the `cn()` utility from the project for conditional or merged class names. Don't write manual ternaries in className strings.
**Incorrect:**
```tsx
<div className={`flex items-center ${isActive ? "bg-primary text-primary-foreground" : "bg-muted"}`}>
```
**Correct:**
```tsx
import { cn } from "@/lib/utils"
<div className={cn("flex items-center", isActive ? "bg-primary text-primary-foreground" : "bg-muted")}>
```
---
## No manual z-index on overlay components
`Dialog`, `Sheet`, `Drawer`, `AlertDialog`, `DropdownMenu`, `Popover`, `Tooltip`, `HoverCard` handle their own stacking. Never add `z-50` or `z-[999]`.

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# React Best Practices
A structured repository for creating and maintaining React Best Practices optimized for agents and LLMs.
## Structure
- `rules/` - Individual rule files (one per rule)
- `_sections.md` - Section metadata (titles, impacts, descriptions)
- `_template.md` - Template for creating new rules
- `area-description.md` - Individual rule files
- `src/` - Build scripts and utilities
- `metadata.json` - Document metadata (version, organization, abstract)
- __`AGENTS.md`__ - Compiled output (generated)
- __`test-cases.json`__ - Test cases for LLM evaluation (generated)
## Getting Started
1. Install dependencies:
```bash
pnpm install
```
2. Build AGENTS.md from rules:
```bash
pnpm build
```
3. Validate rule files:
```bash
pnpm validate
```
4. Extract test cases:
```bash
pnpm extract-tests
```
## Creating a New Rule
1. Copy `rules/_template.md` to `rules/area-description.md`
2. Choose the appropriate area prefix:
- `async-` for Eliminating Waterfalls (Section 1)
- `bundle-` for Bundle Size Optimization (Section 2)
- `server-` for Server-Side Performance (Section 3)
- `client-` for Client-Side Data Fetching (Section 4)
- `rerender-` for Re-render Optimization (Section 5)
- `rendering-` for Rendering Performance (Section 6)
- `js-` for JavaScript Performance (Section 7)
- `advanced-` for Advanced Patterns (Section 8)
3. Fill in the frontmatter and content
4. Ensure you have clear examples with explanations
5. Run `pnpm build` to regenerate AGENTS.md and test-cases.json
## Rule File Structure
Each rule file should follow this structure:
```markdown
---
title: Rule Title Here
impact: MEDIUM
impactDescription: Optional description
tags: tag1, tag2, tag3
---
## Rule Title Here
Brief explanation of the rule and why it matters.
**Incorrect (description of what's wrong):**
```typescript
// Bad code example
```
**Correct (description of what's right):**
```typescript
// Good code example
```
Optional explanatory text after examples.
Reference: [Link](https://example.com)
## File Naming Convention
- Files starting with `_` are special (excluded from build)
- Rule files: `area-description.md` (e.g., `async-parallel.md`)
- Section is automatically inferred from filename prefix
- Rules are sorted alphabetically by title within each section
- IDs (e.g., 1.1, 1.2) are auto-generated during build
## Impact Levels
- `CRITICAL` - Highest priority, major performance gains
- `HIGH` - Significant performance improvements
- `MEDIUM-HIGH` - Moderate-high gains
- `MEDIUM` - Moderate performance improvements
- `LOW-MEDIUM` - Low-medium gains
- `LOW` - Incremental improvements
## Scripts
- `pnpm build` - Compile rules into AGENTS.md
- `pnpm validate` - Validate all rule files
- `pnpm extract-tests` - Extract test cases for LLM evaluation
- `pnpm dev` - Build and validate
## Contributing
When adding or modifying rules:
1. Use the correct filename prefix for your section
2. Follow the `_template.md` structure
3. Include clear bad/good examples with explanations
4. Add appropriate tags
5. Run `pnpm build` to regenerate AGENTS.md and test-cases.json
6. Rules are automatically sorted by title - no need to manage numbers!
## Acknowledgments
Originally created by [@shuding](https://x.com/shuding) at [Vercel](https://vercel.com).

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---
name: vercel-react-best-practices
description: React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
license: MIT
metadata:
author: vercel
version: "1.0.0"
---
# Vercel React Best Practices
Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 58 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
## When to Apply
Reference these guidelines when:
- Writing new React components or Next.js pages
- Implementing data fetching (client or server-side)
- Reviewing code for performance issues
- Refactoring existing React/Next.js code
- Optimizing bundle size or load times
## Rule Categories by Priority
| Priority | Category | Impact | Prefix |
|----------|----------|--------|--------|
| 1 | Eliminating Waterfalls | CRITICAL | `async-` |
| 2 | Bundle Size Optimization | CRITICAL | `bundle-` |
| 3 | Server-Side Performance | HIGH | `server-` |
| 4 | Client-Side Data Fetching | MEDIUM-HIGH | `client-` |
| 5 | Re-render Optimization | MEDIUM | `rerender-` |
| 6 | Rendering Performance | MEDIUM | `rendering-` |
| 7 | JavaScript Performance | LOW-MEDIUM | `js-` |
| 8 | Advanced Patterns | LOW | `advanced-` |
## Quick Reference
### 1. Eliminating Waterfalls (CRITICAL)
- `async-defer-await` - Move await into branches where actually used
- `async-parallel` - Use Promise.all() for independent operations
- `async-dependencies` - Use better-all for partial dependencies
- `async-api-routes` - Start promises early, await late in API routes
- `async-suspense-boundaries` - Use Suspense to stream content
### 2. Bundle Size Optimization (CRITICAL)
- `bundle-barrel-imports` - Import directly, avoid barrel files
- `bundle-dynamic-imports` - Use next/dynamic for heavy components
- `bundle-defer-third-party` - Load analytics/logging after hydration
- `bundle-conditional` - Load modules only when feature is activated
- `bundle-preload` - Preload on hover/focus for perceived speed
### 3. Server-Side Performance (HIGH)
- `server-auth-actions` - Authenticate server actions like API routes
- `server-cache-react` - Use React.cache() for per-request deduplication
- `server-cache-lru` - Use LRU cache for cross-request caching
- `server-dedup-props` - Avoid duplicate serialization in RSC props
- `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
- `server-serialization` - Minimize data passed to client components
- `server-parallel-fetching` - Restructure components to parallelize fetches
- `server-after-nonblocking` - Use after() for non-blocking operations
### 4. Client-Side Data Fetching (MEDIUM-HIGH)
- `client-swr-dedup` - Use SWR for automatic request deduplication
- `client-event-listeners` - Deduplicate global event listeners
- `client-passive-event-listeners` - Use passive listeners for scroll
- `client-localstorage-schema` - Version and minimize localStorage data
### 5. Re-render Optimization (MEDIUM)
- `rerender-defer-reads` - Don't subscribe to state only used in callbacks
- `rerender-memo` - Extract expensive work into memoized components
- `rerender-memo-with-default-value` - Hoist default non-primitive props
- `rerender-dependencies` - Use primitive dependencies in effects
- `rerender-derived-state` - Subscribe to derived booleans, not raw values
- `rerender-derived-state-no-effect` - Derive state during render, not effects
- `rerender-functional-setstate` - Use functional setState for stable callbacks
- `rerender-lazy-state-init` - Pass function to useState for expensive values
- `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
- `rerender-move-effect-to-event` - Put interaction logic in event handlers
- `rerender-transitions` - Use startTransition for non-urgent updates
- `rerender-use-ref-transient-values` - Use refs for transient frequent values
### 6. Rendering Performance (MEDIUM)
- `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element
- `rendering-content-visibility` - Use content-visibility for long lists
- `rendering-hoist-jsx` - Extract static JSX outside components
- `rendering-svg-precision` - Reduce SVG coordinate precision
- `rendering-hydration-no-flicker` - Use inline script for client-only data
- `rendering-hydration-suppress-warning` - Suppress expected mismatches
- `rendering-activity` - Use Activity component for show/hide
- `rendering-conditional-render` - Use ternary, not && for conditionals
- `rendering-usetransition-loading` - Prefer useTransition for loading state
### 7. JavaScript Performance (LOW-MEDIUM)
- `js-batch-dom-css` - Group CSS changes via classes or cssText
- `js-index-maps` - Build Map for repeated lookups
- `js-cache-property-access` - Cache object properties in loops
- `js-cache-function-results` - Cache function results in module-level Map
- `js-cache-storage` - Cache localStorage/sessionStorage reads
- `js-combine-iterations` - Combine multiple filter/map into one loop
- `js-length-check-first` - Check array length before expensive comparison
- `js-early-exit` - Return early from functions
- `js-hoist-regexp` - Hoist RegExp creation outside loops
- `js-min-max-loop` - Use loop for min/max instead of sort
- `js-set-map-lookups` - Use Set/Map for O(1) lookups
- `js-tosorted-immutable` - Use toSorted() for immutability
### 8. Advanced Patterns (LOW)
- `advanced-event-handler-refs` - Store event handlers in refs
- `advanced-init-once` - Initialize app once per app load
- `advanced-use-latest` - useLatest for stable callback refs
## How to Use
Read individual rule files for detailed explanations and code examples:
```
rules/async-parallel.md
rules/bundle-barrel-imports.md
```
Each rule file contains:
- Brief explanation of why it matters
- Incorrect code example with explanation
- Correct code example with explanation
- Additional context and references
## Full Compiled Document
For the complete guide with all rules expanded: `AGENTS.md`

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---
title: Store Event Handlers in Refs
impact: LOW
impactDescription: stable subscriptions
tags: advanced, hooks, refs, event-handlers, optimization
---
## Store Event Handlers in Refs
Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.
**Incorrect (re-subscribes on every render):**
```tsx
function useWindowEvent(event: string, handler: (e) => void) {
useEffect(() => {
window.addEventListener(event, handler)
return () => window.removeEventListener(event, handler)
}, [event, handler])
}
```
**Correct (stable subscription):**
```tsx
function useWindowEvent(event: string, handler: (e) => void) {
const handlerRef = useRef(handler)
useEffect(() => {
handlerRef.current = handler
}, [handler])
useEffect(() => {
const listener = (e) => handlerRef.current(e)
window.addEventListener(event, listener)
return () => window.removeEventListener(event, listener)
}, [event])
}
```
**Alternative: use `useEffectEvent` if you're on latest React:**
```tsx
import { useEffectEvent } from 'react'
function useWindowEvent(event: string, handler: (e) => void) {
const onEvent = useEffectEvent(handler)
useEffect(() => {
window.addEventListener(event, onEvent)
return () => window.removeEventListener(event, onEvent)
}, [event])
}
```
`useEffectEvent` provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.

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---
title: Initialize App Once, Not Per Mount
impact: LOW-MEDIUM
impactDescription: avoids duplicate init in development
tags: initialization, useEffect, app-startup, side-effects
---
## Initialize App Once, Not Per Mount
Do not put app-wide initialization that must run once per app load inside `useEffect([])` of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead.
**Incorrect (runs twice in dev, re-runs on remount):**
```tsx
function Comp() {
useEffect(() => {
loadFromStorage()
checkAuthToken()
}, [])
// ...
}
```
**Correct (once per app load):**
```tsx
let didInit = false
function Comp() {
useEffect(() => {
if (didInit) return
didInit = true
loadFromStorage()
checkAuthToken()
}, [])
// ...
}
```
Reference: [Initializing the application](https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application)

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---
title: useEffectEvent for Stable Callback Refs
impact: LOW
impactDescription: prevents effect re-runs
tags: advanced, hooks, useEffectEvent, refs, optimization
---
## useEffectEvent for Stable Callback Refs
Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
**Incorrect (effect re-runs on every callback change):**
```tsx
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState('')
useEffect(() => {
const timeout = setTimeout(() => onSearch(query), 300)
return () => clearTimeout(timeout)
}, [query, onSearch])
}
```
**Correct (using React's useEffectEvent):**
```tsx
import { useEffectEvent } from 'react';
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState('')
const onSearchEvent = useEffectEvent(onSearch)
useEffect(() => {
const timeout = setTimeout(() => onSearchEvent(query), 300)
return () => clearTimeout(timeout)
}, [query])
}
```

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---
title: Prevent Waterfall Chains in API Routes
impact: CRITICAL
impactDescription: 2-10× improvement
tags: api-routes, server-actions, waterfalls, parallelization
---
## Prevent Waterfall Chains in API Routes
In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.
**Incorrect (config waits for auth, data waits for both):**
```typescript
export async function GET(request: Request) {
const session = await auth()
const config = await fetchConfig()
const data = await fetchData(session.user.id)
return Response.json({ data, config })
}
```
**Correct (auth and config start immediately):**
```typescript
export async function GET(request: Request) {
const sessionPromise = auth()
const configPromise = fetchConfig()
const session = await sessionPromise
const [config, data] = await Promise.all([
configPromise,
fetchData(session.user.id)
])
return Response.json({ data, config })
}
```
For operations with more complex dependency chains, use `better-all` to automatically maximize parallelism (see Dependency-Based Parallelization).

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---
title: Defer Await Until Needed
impact: HIGH
impactDescription: avoids blocking unused code paths
tags: async, await, conditional, optimization
---
## Defer Await Until Needed
Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them.
**Incorrect (blocks both branches):**
```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
const userData = await fetchUserData(userId)
if (skipProcessing) {
// Returns immediately but still waited for userData
return { skipped: true }
}
// Only this branch uses userData
return processUserData(userData)
}
```
**Correct (only blocks when needed):**
```typescript
async function handleRequest(userId: string, skipProcessing: boolean) {
if (skipProcessing) {
// Returns immediately without waiting
return { skipped: true }
}
// Fetch only when needed
const userData = await fetchUserData(userId)
return processUserData(userData)
}
```
**Another example (early return optimization):**
```typescript
// Incorrect: always fetches permissions
async function updateResource(resourceId: string, userId: string) {
const permissions = await fetchPermissions(userId)
const resource = await getResource(resourceId)
if (!resource) {
return { error: 'Not found' }
}
if (!permissions.canEdit) {
return { error: 'Forbidden' }
}
return await updateResourceData(resource, permissions)
}
// Correct: fetches only when needed
async function updateResource(resourceId: string, userId: string) {
const resource = await getResource(resourceId)
if (!resource) {
return { error: 'Not found' }
}
const permissions = await fetchPermissions(userId)
if (!permissions.canEdit) {
return { error: 'Forbidden' }
}
return await updateResourceData(resource, permissions)
}
```
This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.

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---
title: Dependency-Based Parallelization
impact: CRITICAL
impactDescription: 2-10× improvement
tags: async, parallelization, dependencies, better-all
---
## Dependency-Based Parallelization
For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment.
**Incorrect (profile waits for config unnecessarily):**
```typescript
const [user, config] = await Promise.all([
fetchUser(),
fetchConfig()
])
const profile = await fetchProfile(user.id)
```
**Correct (config and profile run in parallel):**
```typescript
import { all } from 'better-all'
const { user, config, profile } = await all({
async user() { return fetchUser() },
async config() { return fetchConfig() },
async profile() {
return fetchProfile((await this.$.user).id)
}
})
```
**Alternative without extra dependencies:**
We can also create all the promises first, and do `Promise.all()` at the end.
```typescript
const userPromise = fetchUser()
const profilePromise = userPromise.then(user => fetchProfile(user.id))
const [user, config, profile] = await Promise.all([
userPromise,
fetchConfig(),
profilePromise
])
```
Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)

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---
title: Promise.all() for Independent Operations
impact: CRITICAL
impactDescription: 2-10× improvement
tags: async, parallelization, promises, waterfalls
---
## Promise.all() for Independent Operations
When async operations have no interdependencies, execute them concurrently using `Promise.all()`.
**Incorrect (sequential execution, 3 round trips):**
```typescript
const user = await fetchUser()
const posts = await fetchPosts()
const comments = await fetchComments()
```
**Correct (parallel execution, 1 round trip):**
```typescript
const [user, posts, comments] = await Promise.all([
fetchUser(),
fetchPosts(),
fetchComments()
])
```

99
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---
title: Strategic Suspense Boundaries
impact: HIGH
impactDescription: faster initial paint
tags: async, suspense, streaming, layout-shift
---
## Strategic Suspense Boundaries
Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.
**Incorrect (wrapper blocked by data fetching):**
```tsx
async function Page() {
const data = await fetchData() // Blocks entire page
return (
<div>
<div>Sidebar</div>
<div>Header</div>
<div>
<DataDisplay data={data} />
</div>
<div>Footer</div>
</div>
)
}
```
The entire layout waits for data even though only the middle section needs it.
**Correct (wrapper shows immediately, data streams in):**
```tsx
function Page() {
return (
<div>
<div>Sidebar</div>
<div>Header</div>
<div>
<Suspense fallback={<Skeleton />}>
<DataDisplay />
</Suspense>
</div>
<div>Footer</div>
</div>
)
}
async function DataDisplay() {
const data = await fetchData() // Only blocks this component
return <div>{data.content}</div>
}
```
Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data.
**Alternative (share promise across components):**
```tsx
function Page() {
// Start fetch immediately, but don't await
const dataPromise = fetchData()
return (
<div>
<div>Sidebar</div>
<div>Header</div>
<Suspense fallback={<Skeleton />}>
<DataDisplay dataPromise={dataPromise} />
<DataSummary dataPromise={dataPromise} />
</Suspense>
<div>Footer</div>
</div>
)
}
function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
const data = use(dataPromise) // Unwraps the promise
return <div>{data.content}</div>
}
function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
const data = use(dataPromise) // Reuses the same promise
return <div>{data.summary}</div>
}
```
Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together.
**When NOT to use this pattern:**
- Critical data needed for layout decisions (affects positioning)
- SEO-critical content above the fold
- Small, fast queries where suspense overhead isn't worth it
- When you want to avoid layout shift (loading → content jump)
**Trade-off:** Faster initial paint vs potential layout shift. Choose based on your UX priorities.

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---
title: Avoid Barrel File Imports
impact: CRITICAL
impactDescription: 200-800ms import cost, slow builds
tags: bundle, imports, tree-shaking, barrel-files, performance
---
## Avoid Barrel File Imports
Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`).
Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts.
**Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph.
**Incorrect (imports entire library):**
```tsx
import { Check, X, Menu } from 'lucide-react'
// Loads 1,583 modules, takes ~2.8s extra in dev
// Runtime cost: 200-800ms on every cold start
import { Button, TextField } from '@mui/material'
// Loads 2,225 modules, takes ~4.2s extra in dev
```
**Correct (imports only what you need):**
```tsx
import Check from 'lucide-react/dist/esm/icons/check'
import X from 'lucide-react/dist/esm/icons/x'
import Menu from 'lucide-react/dist/esm/icons/menu'
// Loads only 3 modules (~2KB vs ~1MB)
import Button from '@mui/material/Button'
import TextField from '@mui/material/TextField'
// Loads only what you use
```
**Alternative (Next.js 13.5+):**
```js
// next.config.js - use optimizePackageImports
module.exports = {
experimental: {
optimizePackageImports: ['lucide-react', '@mui/material']
}
}
// Then you can keep the ergonomic barrel imports:
import { Check, X, Menu } from 'lucide-react'
// Automatically transformed to direct imports at build time
```
Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR.
Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`.
Reference: [How we optimized package imports in Next.js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)

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---
title: Conditional Module Loading
impact: HIGH
impactDescription: loads large data only when needed
tags: bundle, conditional-loading, lazy-loading
---
## Conditional Module Loading
Load large data or modules only when a feature is activated.
**Example (lazy-load animation frames):**
```tsx
function AnimationPlayer({ enabled, setEnabled }: { enabled: boolean; setEnabled: React.Dispatch<React.SetStateAction<boolean>> }) {
const [frames, setFrames] = useState<Frame[] | null>(null)
useEffect(() => {
if (enabled && !frames && typeof window !== 'undefined') {
import('./animation-frames.js')
.then(mod => setFrames(mod.frames))
.catch(() => setEnabled(false))
}
}, [enabled, frames, setEnabled])
if (!frames) return <Skeleton />
return <Canvas frames={frames} />
}
```
The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed.

49
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---
title: Defer Non-Critical Third-Party Libraries
impact: MEDIUM
impactDescription: loads after hydration
tags: bundle, third-party, analytics, defer
---
## Defer Non-Critical Third-Party Libraries
Analytics, logging, and error tracking don't block user interaction. Load them after hydration.
**Incorrect (blocks initial bundle):**
```tsx
import { Analytics } from '@vercel/analytics/react'
export default function RootLayout({ children }) {
return (
<html>
<body>
{children}
<Analytics />
</body>
</html>
)
}
```
**Correct (loads after hydration):**
```tsx
import dynamic from 'next/dynamic'
const Analytics = dynamic(
() => import('@vercel/analytics/react').then(m => m.Analytics),
{ ssr: false }
)
export default function RootLayout({ children }) {
return (
<html>
<body>
{children}
<Analytics />
</body>
</html>
)
}
```

35
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---
title: Dynamic Imports for Heavy Components
impact: CRITICAL
impactDescription: directly affects TTI and LCP
tags: bundle, dynamic-import, code-splitting, next-dynamic
---
## Dynamic Imports for Heavy Components
Use `next/dynamic` to lazy-load large components not needed on initial render.
**Incorrect (Monaco bundles with main chunk ~300KB):**
```tsx
import { MonacoEditor } from './monaco-editor'
function CodePanel({ code }: { code: string }) {
return <MonacoEditor value={code} />
}
```
**Correct (Monaco loads on demand):**
```tsx
import dynamic from 'next/dynamic'
const MonacoEditor = dynamic(
() => import('./monaco-editor').then(m => m.MonacoEditor),
{ ssr: false }
)
function CodePanel({ code }: { code: string }) {
return <MonacoEditor value={code} />
}
```

50
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---
title: Preload Based on User Intent
impact: MEDIUM
impactDescription: reduces perceived latency
tags: bundle, preload, user-intent, hover
---
## Preload Based on User Intent
Preload heavy bundles before they're needed to reduce perceived latency.
**Example (preload on hover/focus):**
```tsx
function EditorButton({ onClick }: { onClick: () => void }) {
const preload = () => {
if (typeof window !== 'undefined') {
void import('./monaco-editor')
}
}
return (
<button
onMouseEnter={preload}
onFocus={preload}
onClick={onClick}
>
Open Editor
</button>
)
}
```
**Example (preload when feature flag is enabled):**
```tsx
function FlagsProvider({ children, flags }: Props) {
useEffect(() => {
if (flags.editorEnabled && typeof window !== 'undefined') {
void import('./monaco-editor').then(mod => mod.init())
}
}, [flags.editorEnabled])
return <FlagsContext.Provider value={flags}>
{children}
</FlagsContext.Provider>
}
```
The `typeof window !== 'undefined'` check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.

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---
title: Deduplicate Global Event Listeners
impact: LOW
impactDescription: single listener for N components
tags: client, swr, event-listeners, subscription
---
## Deduplicate Global Event Listeners
Use `useSWRSubscription()` to share global event listeners across component instances.
**Incorrect (N instances = N listeners):**
```tsx
function useKeyboardShortcut(key: string, callback: () => void) {
useEffect(() => {
const handler = (e: KeyboardEvent) => {
if (e.metaKey && e.key === key) {
callback()
}
}
window.addEventListener('keydown', handler)
return () => window.removeEventListener('keydown', handler)
}, [key, callback])
}
```
When using the `useKeyboardShortcut` hook multiple times, each instance will register a new listener.
**Correct (N instances = 1 listener):**
```tsx
import useSWRSubscription from 'swr/subscription'
// Module-level Map to track callbacks per key
const keyCallbacks = new Map<string, Set<() => void>>()
function useKeyboardShortcut(key: string, callback: () => void) {
// Register this callback in the Map
useEffect(() => {
if (!keyCallbacks.has(key)) {
keyCallbacks.set(key, new Set())
}
keyCallbacks.get(key)!.add(callback)
return () => {
const set = keyCallbacks.get(key)
if (set) {
set.delete(callback)
if (set.size === 0) {
keyCallbacks.delete(key)
}
}
}
}, [key, callback])
useSWRSubscription('global-keydown', () => {
const handler = (e: KeyboardEvent) => {
if (e.metaKey && keyCallbacks.has(e.key)) {
keyCallbacks.get(e.key)!.forEach(cb => cb())
}
}
window.addEventListener('keydown', handler)
return () => window.removeEventListener('keydown', handler)
})
}
function Profile() {
// Multiple shortcuts will share the same listener
useKeyboardShortcut('p', () => { /* ... */ })
useKeyboardShortcut('k', () => { /* ... */ })
// ...
}
```

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---
title: Version and Minimize localStorage Data
impact: MEDIUM
impactDescription: prevents schema conflicts, reduces storage size
tags: client, localStorage, storage, versioning, data-minimization
---
## Version and Minimize localStorage Data
Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data.
**Incorrect:**
```typescript
// No version, stores everything, no error handling
localStorage.setItem('userConfig', JSON.stringify(fullUserObject))
const data = localStorage.getItem('userConfig')
```
**Correct:**
```typescript
const VERSION = 'v2'
function saveConfig(config: { theme: string; language: string }) {
try {
localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config))
} catch {
// Throws in incognito/private browsing, quota exceeded, or disabled
}
}
function loadConfig() {
try {
const data = localStorage.getItem(`userConfig:${VERSION}`)
return data ? JSON.parse(data) : null
} catch {
return null
}
}
// Migration from v1 to v2
function migrate() {
try {
const v1 = localStorage.getItem('userConfig:v1')
if (v1) {
const old = JSON.parse(v1)
saveConfig({ theme: old.darkMode ? 'dark' : 'light', language: old.lang })
localStorage.removeItem('userConfig:v1')
}
} catch {}
}
```
**Store minimal fields from server responses:**
```typescript
// User object has 20+ fields, only store what UI needs
function cachePrefs(user: FullUser) {
try {
localStorage.setItem('prefs:v1', JSON.stringify({
theme: user.preferences.theme,
notifications: user.preferences.notifications
}))
} catch {}
}
```
**Always wrap in try-catch:** `getItem()` and `setItem()` throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled.
**Benefits:** Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags.

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---
title: Use Passive Event Listeners for Scrolling Performance
impact: MEDIUM
impactDescription: eliminates scroll delay caused by event listeners
tags: client, event-listeners, scrolling, performance, touch, wheel
---
## Use Passive Event Listeners for Scrolling Performance
Add `{ passive: true }` to touch and wheel event listeners to enable immediate scrolling. Browsers normally wait for listeners to finish to check if `preventDefault()` is called, causing scroll delay.
**Incorrect:**
```typescript
useEffect(() => {
const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX)
const handleWheel = (e: WheelEvent) => console.log(e.deltaY)
document.addEventListener('touchstart', handleTouch)
document.addEventListener('wheel', handleWheel)
return () => {
document.removeEventListener('touchstart', handleTouch)
document.removeEventListener('wheel', handleWheel)
}
}, [])
```
**Correct:**
```typescript
useEffect(() => {
const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX)
const handleWheel = (e: WheelEvent) => console.log(e.deltaY)
document.addEventListener('touchstart', handleTouch, { passive: true })
document.addEventListener('wheel', handleWheel, { passive: true })
return () => {
document.removeEventListener('touchstart', handleTouch)
document.removeEventListener('wheel', handleWheel)
}
}, [])
```
**Use passive when:** tracking/analytics, logging, any listener that doesn't call `preventDefault()`.
**Don't use passive when:** implementing custom swipe gestures, custom zoom controls, or any listener that needs `preventDefault()`.

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---
title: Use SWR for Automatic Deduplication
impact: MEDIUM-HIGH
impactDescription: automatic deduplication
tags: client, swr, deduplication, data-fetching
---
## Use SWR for Automatic Deduplication
SWR enables request deduplication, caching, and revalidation across component instances.
**Incorrect (no deduplication, each instance fetches):**
```tsx
function UserList() {
const [users, setUsers] = useState([])
useEffect(() => {
fetch('/api/users')
.then(r => r.json())
.then(setUsers)
}, [])
}
```
**Correct (multiple instances share one request):**
```tsx
import useSWR from 'swr'
function UserList() {
const { data: users } = useSWR('/api/users', fetcher)
}
```
**For immutable data:**
```tsx
import { useImmutableSWR } from '@/lib/swr'
function StaticContent() {
const { data } = useImmutableSWR('/api/config', fetcher)
}
```
**For mutations:**
```tsx
import { useSWRMutation } from 'swr/mutation'
function UpdateButton() {
const { trigger } = useSWRMutation('/api/user', updateUser)
return <button onClick={() => trigger()}>Update</button>
}
```
Reference: [https://swr.vercel.app](https://swr.vercel.app)

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---
title: Avoid Layout Thrashing
impact: MEDIUM
impactDescription: prevents forced synchronous layouts and reduces performance bottlenecks
tags: javascript, dom, css, performance, reflow, layout-thrashing
---
## Avoid Layout Thrashing
Avoid interleaving style writes with layout reads. When you read a layout property (like `offsetWidth`, `getBoundingClientRect()`, or `getComputedStyle()`) between style changes, the browser is forced to trigger a synchronous reflow.
**This is OK (browser batches style changes):**
```typescript
function updateElementStyles(element: HTMLElement) {
// Each line invalidates style, but browser batches the recalculation
element.style.width = '100px'
element.style.height = '200px'
element.style.backgroundColor = 'blue'
element.style.border = '1px solid black'
}
```
**Incorrect (interleaved reads and writes force reflows):**
```typescript
function layoutThrashing(element: HTMLElement) {
element.style.width = '100px'
const width = element.offsetWidth // Forces reflow
element.style.height = '200px'
const height = element.offsetHeight // Forces another reflow
}
```
**Correct (batch writes, then read once):**
```typescript
function updateElementStyles(element: HTMLElement) {
// Batch all writes together
element.style.width = '100px'
element.style.height = '200px'
element.style.backgroundColor = 'blue'
element.style.border = '1px solid black'
// Read after all writes are done (single reflow)
const { width, height } = element.getBoundingClientRect()
}
```
**Correct (batch reads, then writes):**
```typescript
function avoidThrashing(element: HTMLElement) {
// Read phase - all layout queries first
const rect1 = element.getBoundingClientRect()
const offsetWidth = element.offsetWidth
const offsetHeight = element.offsetHeight
// Write phase - all style changes after
element.style.width = '100px'
element.style.height = '200px'
}
```
**Better: use CSS classes**
```css
.highlighted-box {
width: 100px;
height: 200px;
background-color: blue;
border: 1px solid black;
}
```
```typescript
function updateElementStyles(element: HTMLElement) {
element.classList.add('highlighted-box')
const { width, height } = element.getBoundingClientRect()
}
```
**React example:**
```tsx
// Incorrect: interleaving style changes with layout queries
function Box({ isHighlighted }: { isHighlighted: boolean }) {
const ref = useRef<HTMLDivElement>(null)
useEffect(() => {
if (ref.current && isHighlighted) {
ref.current.style.width = '100px'
const width = ref.current.offsetWidth // Forces layout
ref.current.style.height = '200px'
}
}, [isHighlighted])
return <div ref={ref}>Content</div>
}
// Correct: toggle class
function Box({ isHighlighted }: { isHighlighted: boolean }) {
return (
<div className={isHighlighted ? 'highlighted-box' : ''}>
Content
</div>
)
}
```
Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.
See [this gist](https://gist.github.com/paulirish/5d52fb081b3570c81e3a) and [CSS Triggers](https://csstriggers.com/) for more information on layout-forcing operations.

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---
title: Cache Repeated Function Calls
impact: MEDIUM
impactDescription: avoid redundant computation
tags: javascript, cache, memoization, performance
---
## Cache Repeated Function Calls
Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
**Incorrect (redundant computation):**
```typescript
function ProjectList({ projects }: { projects: Project[] }) {
return (
<div>
{projects.map(project => {
// slugify() called 100+ times for same project names
const slug = slugify(project.name)
return <ProjectCard key={project.id} slug={slug} />
})}
</div>
)
}
```
**Correct (cached results):**
```typescript
// Module-level cache
const slugifyCache = new Map<string, string>()
function cachedSlugify(text: string): string {
if (slugifyCache.has(text)) {
return slugifyCache.get(text)!
}
const result = slugify(text)
slugifyCache.set(text, result)
return result
}
function ProjectList({ projects }: { projects: Project[] }) {
return (
<div>
{projects.map(project => {
// Computed only once per unique project name
const slug = cachedSlugify(project.name)
return <ProjectCard key={project.id} slug={slug} />
})}
</div>
)
}
```
**Simpler pattern for single-value functions:**
```typescript
let isLoggedInCache: boolean | null = null
function isLoggedIn(): boolean {
if (isLoggedInCache !== null) {
return isLoggedInCache
}
isLoggedInCache = document.cookie.includes('auth=')
return isLoggedInCache
}
// Clear cache when auth changes
function onAuthChange() {
isLoggedInCache = null
}
```
Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
Reference: [How we made the Vercel Dashboard twice as fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)

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---
title: Cache Property Access in Loops
impact: LOW-MEDIUM
impactDescription: reduces lookups
tags: javascript, loops, optimization, caching
---
## Cache Property Access in Loops
Cache object property lookups in hot paths.
**Incorrect (3 lookups × N iterations):**
```typescript
for (let i = 0; i < arr.length; i++) {
process(obj.config.settings.value)
}
```
**Correct (1 lookup total):**
```typescript
const value = obj.config.settings.value
const len = arr.length
for (let i = 0; i < len; i++) {
process(value)
}
```

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---
title: Cache Storage API Calls
impact: LOW-MEDIUM
impactDescription: reduces expensive I/O
tags: javascript, localStorage, storage, caching, performance
---
## Cache Storage API Calls
`localStorage`, `sessionStorage`, and `document.cookie` are synchronous and expensive. Cache reads in memory.
**Incorrect (reads storage on every call):**
```typescript
function getTheme() {
return localStorage.getItem('theme') ?? 'light'
}
// Called 10 times = 10 storage reads
```
**Correct (Map cache):**
```typescript
const storageCache = new Map<string, string | null>()
function getLocalStorage(key: string) {
if (!storageCache.has(key)) {
storageCache.set(key, localStorage.getItem(key))
}
return storageCache.get(key)
}
function setLocalStorage(key: string, value: string) {
localStorage.setItem(key, value)
storageCache.set(key, value) // keep cache in sync
}
```
Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
**Cookie caching:**
```typescript
let cookieCache: Record<string, string> | null = null
function getCookie(name: string) {
if (!cookieCache) {
cookieCache = Object.fromEntries(
document.cookie.split('; ').map(c => c.split('='))
)
}
return cookieCache[name]
}
```
**Important (invalidate on external changes):**
If storage can change externally (another tab, server-set cookies), invalidate cache:
```typescript
window.addEventListener('storage', (e) => {
if (e.key) storageCache.delete(e.key)
})
document.addEventListener('visibilitychange', () => {
if (document.visibilityState === 'visible') {
storageCache.clear()
}
})
```

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---
title: Combine Multiple Array Iterations
impact: LOW-MEDIUM
impactDescription: reduces iterations
tags: javascript, arrays, loops, performance
---
## Combine Multiple Array Iterations
Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop.
**Incorrect (3 iterations):**
```typescript
const admins = users.filter(u => u.isAdmin)
const testers = users.filter(u => u.isTester)
const inactive = users.filter(u => !u.isActive)
```
**Correct (1 iteration):**
```typescript
const admins: User[] = []
const testers: User[] = []
const inactive: User[] = []
for (const user of users) {
if (user.isAdmin) admins.push(user)
if (user.isTester) testers.push(user)
if (!user.isActive) inactive.push(user)
}
```

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---
title: Early Return from Functions
impact: LOW-MEDIUM
impactDescription: avoids unnecessary computation
tags: javascript, functions, optimization, early-return
---
## Early Return from Functions
Return early when result is determined to skip unnecessary processing.
**Incorrect (processes all items even after finding answer):**
```typescript
function validateUsers(users: User[]) {
let hasError = false
let errorMessage = ''
for (const user of users) {
if (!user.email) {
hasError = true
errorMessage = 'Email required'
}
if (!user.name) {
hasError = true
errorMessage = 'Name required'
}
// Continues checking all users even after error found
}
return hasError ? { valid: false, error: errorMessage } : { valid: true }
}
```
**Correct (returns immediately on first error):**
```typescript
function validateUsers(users: User[]) {
for (const user of users) {
if (!user.email) {
return { valid: false, error: 'Email required' }
}
if (!user.name) {
return { valid: false, error: 'Name required' }
}
}
return { valid: true }
}
```

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---
title: Hoist RegExp Creation
impact: LOW-MEDIUM
impactDescription: avoids recreation
tags: javascript, regexp, optimization, memoization
---
## Hoist RegExp Creation
Don't create RegExp inside render. Hoist to module scope or memoize with `useMemo()`.
**Incorrect (new RegExp every render):**
```tsx
function Highlighter({ text, query }: Props) {
const regex = new RegExp(`(${query})`, 'gi')
const parts = text.split(regex)
return <>{parts.map((part, i) => ...)}</>
}
```
**Correct (memoize or hoist):**
```tsx
const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
function Highlighter({ text, query }: Props) {
const regex = useMemo(
() => new RegExp(`(${escapeRegex(query)})`, 'gi'),
[query]
)
const parts = text.split(regex)
return <>{parts.map((part, i) => ...)}</>
}
```
**Warning (global regex has mutable state):**
Global regex (`/g`) has mutable `lastIndex` state:
```typescript
const regex = /foo/g
regex.test('foo') // true, lastIndex = 3
regex.test('foo') // false, lastIndex = 0
```

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---
title: Build Index Maps for Repeated Lookups
impact: LOW-MEDIUM
impactDescription: 1M ops to 2K ops
tags: javascript, map, indexing, optimization, performance
---
## Build Index Maps for Repeated Lookups
Multiple `.find()` calls by the same key should use a Map.
**Incorrect (O(n) per lookup):**
```typescript
function processOrders(orders: Order[], users: User[]) {
return orders.map(order => ({
...order,
user: users.find(u => u.id === order.userId)
}))
}
```
**Correct (O(1) per lookup):**
```typescript
function processOrders(orders: Order[], users: User[]) {
const userById = new Map(users.map(u => [u.id, u]))
return orders.map(order => ({
...order,
user: userById.get(order.userId)
}))
}
```
Build map once (O(n)), then all lookups are O(1).
For 1000 orders × 1000 users: 1M ops → 2K ops.

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---
title: Early Length Check for Array Comparisons
impact: MEDIUM-HIGH
impactDescription: avoids expensive operations when lengths differ
tags: javascript, arrays, performance, optimization, comparison
---
## Early Length Check for Array Comparisons
When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
**Incorrect (always runs expensive comparison):**
```typescript
function hasChanges(current: string[], original: string[]) {
// Always sorts and joins, even when lengths differ
return current.sort().join() !== original.sort().join()
}
```
Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings.
**Correct (O(1) length check first):**
```typescript
function hasChanges(current: string[], original: string[]) {
// Early return if lengths differ
if (current.length !== original.length) {
return true
}
// Only sort when lengths match
const currentSorted = current.toSorted()
const originalSorted = original.toSorted()
for (let i = 0; i < currentSorted.length; i++) {
if (currentSorted[i] !== originalSorted[i]) {
return true
}
}
return false
}
```
This new approach is more efficient because:
- It avoids the overhead of sorting and joining the arrays when lengths differ
- It avoids consuming memory for the joined strings (especially important for large arrays)
- It avoids mutating the original arrays
- It returns early when a difference is found

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---
title: Use Loop for Min/Max Instead of Sort
impact: LOW
impactDescription: O(n) instead of O(n log n)
tags: javascript, arrays, performance, sorting, algorithms
---
## Use Loop for Min/Max Instead of Sort
Finding the smallest or largest element only requires a single pass through the array. Sorting is wasteful and slower.
**Incorrect (O(n log n) - sort to find latest):**
```typescript
interface Project {
id: string
name: string
updatedAt: number
}
function getLatestProject(projects: Project[]) {
const sorted = [...projects].sort((a, b) => b.updatedAt - a.updatedAt)
return sorted[0]
}
```
Sorts the entire array just to find the maximum value.
**Incorrect (O(n log n) - sort for oldest and newest):**
```typescript
function getOldestAndNewest(projects: Project[]) {
const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt)
return { oldest: sorted[0], newest: sorted[sorted.length - 1] }
}
```
Still sorts unnecessarily when only min/max are needed.
**Correct (O(n) - single loop):**
```typescript
function getLatestProject(projects: Project[]) {
if (projects.length === 0) return null
let latest = projects[0]
for (let i = 1; i < projects.length; i++) {
if (projects[i].updatedAt > latest.updatedAt) {
latest = projects[i]
}
}
return latest
}
function getOldestAndNewest(projects: Project[]) {
if (projects.length === 0) return { oldest: null, newest: null }
let oldest = projects[0]
let newest = projects[0]
for (let i = 1; i < projects.length; i++) {
if (projects[i].updatedAt < oldest.updatedAt) oldest = projects[i]
if (projects[i].updatedAt > newest.updatedAt) newest = projects[i]
}
return { oldest, newest }
}
```
Single pass through the array, no copying, no sorting.
**Alternative (Math.min/Math.max for small arrays):**
```typescript
const numbers = [5, 2, 8, 1, 9]
const min = Math.min(...numbers)
const max = Math.max(...numbers)
```
This works for small arrays, but can be slower or just throw an error for very large arrays due to spread operator limitations. Maximal array length is approximately 124000 in Chrome 143 and 638000 in Safari 18; exact numbers may vary - see [the fiddle](https://jsfiddle.net/qw1jabsx/4/). Use the loop approach for reliability.

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---
title: Use Set/Map for O(1) Lookups
impact: LOW-MEDIUM
impactDescription: O(n) to O(1)
tags: javascript, set, map, data-structures, performance
---
## Use Set/Map for O(1) Lookups
Convert arrays to Set/Map for repeated membership checks.
**Incorrect (O(n) per check):**
```typescript
const allowedIds = ['a', 'b', 'c', ...]
items.filter(item => allowedIds.includes(item.id))
```
**Correct (O(1) per check):**
```typescript
const allowedIds = new Set(['a', 'b', 'c', ...])
items.filter(item => allowedIds.has(item.id))
```

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---
title: Use toSorted() Instead of sort() for Immutability
impact: MEDIUM-HIGH
impactDescription: prevents mutation bugs in React state
tags: javascript, arrays, immutability, react, state, mutation
---
## Use toSorted() Instead of sort() for Immutability
`.sort()` mutates the array in place, which can cause bugs with React state and props. Use `.toSorted()` to create a new sorted array without mutation.
**Incorrect (mutates original array):**
```typescript
function UserList({ users }: { users: User[] }) {
// Mutates the users prop array!
const sorted = useMemo(
() => users.sort((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
```
**Correct (creates new array):**
```typescript
function UserList({ users }: { users: User[] }) {
// Creates new sorted array, original unchanged
const sorted = useMemo(
() => users.toSorted((a, b) => a.name.localeCompare(b.name)),
[users]
)
return <div>{sorted.map(renderUser)}</div>
}
```
**Why this matters in React:**
1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only
2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior
**Browser support (fallback for older browsers):**
`.toSorted()` is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:
```typescript
// Fallback for older browsers
const sorted = [...items].sort((a, b) => a.value - b.value)
```
**Other immutable array methods:**
- `.toSorted()` - immutable sort
- `.toReversed()` - immutable reverse
- `.toSpliced()` - immutable splice
- `.with()` - immutable element replacement

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---
title: Use Activity Component for Show/Hide
impact: MEDIUM
impactDescription: preserves state/DOM
tags: rendering, activity, visibility, state-preservation
---
## Use Activity Component for Show/Hide
Use React's `<Activity>` to preserve state/DOM for expensive components that frequently toggle visibility.
**Usage:**
```tsx
import { Activity } from 'react'
function Dropdown({ isOpen }: Props) {
return (
<Activity mode={isOpen ? 'visible' : 'hidden'}>
<ExpensiveMenu />
</Activity>
)
}
```
Avoids expensive re-renders and state loss.

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---
title: Animate SVG Wrapper Instead of SVG Element
impact: LOW
impactDescription: enables hardware acceleration
tags: rendering, svg, css, animation, performance
---
## Animate SVG Wrapper Instead of SVG Element
Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a `<div>` and animate the wrapper instead.
**Incorrect (animating SVG directly - no hardware acceleration):**
```tsx
function LoadingSpinner() {
return (
<svg
className="animate-spin"
width="24"
height="24"
viewBox="0 0 24 24"
>
<circle cx="12" cy="12" r="10" stroke="currentColor" />
</svg>
)
}
```
**Correct (animating wrapper div - hardware accelerated):**
```tsx
function LoadingSpinner() {
return (
<div className="animate-spin">
<svg
width="24"
height="24"
viewBox="0 0 24 24"
>
<circle cx="12" cy="12" r="10" stroke="currentColor" />
</svg>
</div>
)
}
```
This applies to all CSS transforms and transitions (`transform`, `opacity`, `translate`, `scale`, `rotate`). The wrapper div allows browsers to use GPU acceleration for smoother animations.

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---
title: Use Explicit Conditional Rendering
impact: LOW
impactDescription: prevents rendering 0 or NaN
tags: rendering, conditional, jsx, falsy-values
---
## Use Explicit Conditional Rendering
Use explicit ternary operators (`? :`) instead of `&&` for conditional rendering when the condition can be `0`, `NaN`, or other falsy values that render.
**Incorrect (renders "0" when count is 0):**
```tsx
function Badge({ count }: { count: number }) {
return (
<div>
{count && <span className="badge">{count}</span>}
</div>
)
}
// When count = 0, renders: <div>0</div>
// When count = 5, renders: <div><span class="badge">5</span></div>
```
**Correct (renders nothing when count is 0):**
```tsx
function Badge({ count }: { count: number }) {
return (
<div>
{count > 0 ? <span className="badge">{count}</span> : null}
</div>
)
}
// When count = 0, renders: <div></div>
// When count = 5, renders: <div><span class="badge">5</span></div>
```

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---
title: CSS content-visibility for Long Lists
impact: HIGH
impactDescription: faster initial render
tags: rendering, css, content-visibility, long-lists
---
## CSS content-visibility for Long Lists
Apply `content-visibility: auto` to defer off-screen rendering.
**CSS:**
```css
.message-item {
content-visibility: auto;
contain-intrinsic-size: 0 80px;
}
```
**Example:**
```tsx
function MessageList({ messages }: { messages: Message[] }) {
return (
<div className="overflow-y-auto h-screen">
{messages.map(msg => (
<div key={msg.id} className="message-item">
<Avatar user={msg.author} />
<div>{msg.content}</div>
</div>
))}
</div>
)
}
```
For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).

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---
title: Hoist Static JSX Elements
impact: LOW
impactDescription: avoids re-creation
tags: rendering, jsx, static, optimization
---
## Hoist Static JSX Elements
Extract static JSX outside components to avoid re-creation.
**Incorrect (recreates element every render):**
```tsx
function LoadingSkeleton() {
return <div className="animate-pulse h-20 bg-gray-200" />
}
function Container() {
return (
<div>
{loading && <LoadingSkeleton />}
</div>
)
}
```
**Correct (reuses same element):**
```tsx
const loadingSkeleton = (
<div className="animate-pulse h-20 bg-gray-200" />
)
function Container() {
return (
<div>
{loading && loadingSkeleton}
</div>
)
}
```
This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.

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---
title: Prevent Hydration Mismatch Without Flickering
impact: MEDIUM
impactDescription: avoids visual flicker and hydration errors
tags: rendering, ssr, hydration, localStorage, flicker
---
## Prevent Hydration Mismatch Without Flickering
When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.
**Incorrect (breaks SSR):**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
// localStorage is not available on server - throws error
const theme = localStorage.getItem('theme') || 'light'
return (
<div className={theme}>
{children}
</div>
)
}
```
Server-side rendering will fail because `localStorage` is undefined.
**Incorrect (visual flickering):**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
const [theme, setTheme] = useState('light')
useEffect(() => {
// Runs after hydration - causes visible flash
const stored = localStorage.getItem('theme')
if (stored) {
setTheme(stored)
}
}, [])
return (
<div className={theme}>
{children}
</div>
)
}
```
Component first renders with default value (`light`), then updates after hydration, causing a visible flash of incorrect content.
**Correct (no flicker, no hydration mismatch):**
```tsx
function ThemeWrapper({ children }: { children: ReactNode }) {
return (
<>
<div id="theme-wrapper">
{children}
</div>
<script
dangerouslySetInnerHTML={{
__html: `
(function() {
try {
var theme = localStorage.getItem('theme') || 'light';
var el = document.getElementById('theme-wrapper');
if (el) el.className = theme;
} catch (e) {}
})();
`,
}}
/>
</>
)
}
```
The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.
This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.

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---
title: Suppress Expected Hydration Mismatches
impact: LOW-MEDIUM
impactDescription: avoids noisy hydration warnings for known differences
tags: rendering, hydration, ssr, nextjs
---
## Suppress Expected Hydration Mismatches
In SSR frameworks (e.g., Next.js), some values are intentionally different on server vs client (random IDs, dates, locale/timezone formatting). For these *expected* mismatches, wrap the dynamic text in an element with `suppressHydrationWarning` to prevent noisy warnings. Do not use this to hide real bugs. Don’t overuse it.
**Incorrect (known mismatch warnings):**
```tsx
function Timestamp() {
return <span>{new Date().toLocaleString()}</span>
}
```
**Correct (suppress expected mismatch only):**
```tsx
function Timestamp() {
return (
<span suppressHydrationWarning>
{new Date().toLocaleString()}
</span>
)
}
```

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---
title: Optimize SVG Precision
impact: LOW
impactDescription: reduces file size
tags: rendering, svg, optimization, svgo
---
## Optimize SVG Precision
Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.
**Incorrect (excessive precision):**
```svg
<path d="M 10.293847 20.847362 L 30.938472 40.192837" />
```
**Correct (1 decimal place):**
```svg
<path d="M 10.3 20.8 L 30.9 40.2" />
```
**Automate with SVGO:**
```bash
npx svgo --precision=1 --multipass icon.svg
```

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---
title: Use useTransition Over Manual Loading States
impact: LOW
impactDescription: reduces re-renders and improves code clarity
tags: rendering, transitions, useTransition, loading, state
---
## Use useTransition Over Manual Loading States
Use `useTransition` instead of manual `useState` for loading states. This provides built-in `isPending` state and automatically manages transitions.
**Incorrect (manual loading state):**
```tsx
function SearchResults() {
const [query, setQuery] = useState('')
const [results, setResults] = useState([])
const [isLoading, setIsLoading] = useState(false)
const handleSearch = async (value: string) => {
setIsLoading(true)
setQuery(value)
const data = await fetchResults(value)
setResults(data)
setIsLoading(false)
}
return (
<>
<input onChange={(e) => handleSearch(e.target.value)} />
{isLoading && <Spinner />}
<ResultsList results={results} />
</>
)
}
```
**Correct (useTransition with built-in pending state):**
```tsx
import { useTransition, useState } from 'react'
function SearchResults() {
const [query, setQuery] = useState('')
const [results, setResults] = useState([])
const [isPending, startTransition] = useTransition()
const handleSearch = (value: string) => {
setQuery(value) // Update input immediately
startTransition(async () => {
// Fetch and update results
const data = await fetchResults(value)
setResults(data)
})
}
return (
<>
<input onChange={(e) => handleSearch(e.target.value)} />
{isPending && <Spinner />}
<ResultsList results={results} />
</>
)
}
```
**Benefits:**
- **Automatic pending state**: No need to manually manage `setIsLoading(true/false)`
- **Error resilience**: Pending state correctly resets even if the transition throws
- **Better responsiveness**: Keeps the UI responsive during updates
- **Interrupt handling**: New transitions automatically cancel pending ones
Reference: [useTransition](https://react.dev/reference/react/useTransition)

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---
title: Defer State Reads to Usage Point
impact: MEDIUM
impactDescription: avoids unnecessary subscriptions
tags: rerender, searchParams, localStorage, optimization
---
## Defer State Reads to Usage Point
Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.
**Incorrect (subscribes to all searchParams changes):**
```tsx
function ShareButton({ chatId }: { chatId: string }) {
const searchParams = useSearchParams()
const handleShare = () => {
const ref = searchParams.get('ref')
shareChat(chatId, { ref })
}
return <button onClick={handleShare}>Share</button>
}
```
**Correct (reads on demand, no subscription):**
```tsx
function ShareButton({ chatId }: { chatId: string }) {
const handleShare = () => {
const params = new URLSearchParams(window.location.search)
const ref = params.get('ref')
shareChat(chatId, { ref })
}
return <button onClick={handleShare}>Share</button>
}
```

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---
title: Narrow Effect Dependencies
impact: LOW
impactDescription: minimizes effect re-runs
tags: rerender, useEffect, dependencies, optimization
---
## Narrow Effect Dependencies
Specify primitive dependencies instead of objects to minimize effect re-runs.
**Incorrect (re-runs on any user field change):**
```tsx
useEffect(() => {
console.log(user.id)
}, [user])
```
**Correct (re-runs only when id changes):**
```tsx
useEffect(() => {
console.log(user.id)
}, [user.id])
```
**For derived state, compute outside effect:**
```tsx
// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
if (width < 768) {
enableMobileMode()
}
}, [width])
// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
if (isMobile) {
enableMobileMode()
}
}, [isMobile])
```

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---
title: Calculate Derived State During Rendering
impact: MEDIUM
impactDescription: avoids redundant renders and state drift
tags: rerender, derived-state, useEffect, state
---
## Calculate Derived State During Rendering
If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.
**Incorrect (redundant state and effect):**
```tsx
function Form() {
const [firstName, setFirstName] = useState('First')
const [lastName, setLastName] = useState('Last')
const [fullName, setFullName] = useState('')
useEffect(() => {
setFullName(firstName + ' ' + lastName)
}, [firstName, lastName])
return <p>{fullName}</p>
}
```
**Correct (derive during render):**
```tsx
function Form() {
const [firstName, setFirstName] = useState('First')
const [lastName, setLastName] = useState('Last')
const fullName = firstName + ' ' + lastName
return <p>{fullName}</p>
}
```
References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)

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---
title: Subscribe to Derived State
impact: MEDIUM
impactDescription: reduces re-render frequency
tags: rerender, derived-state, media-query, optimization
---
## Subscribe to Derived State
Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
**Incorrect (re-renders on every pixel change):**
```tsx
function Sidebar() {
const width = useWindowWidth() // updates continuously
const isMobile = width < 768
return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```
**Correct (re-renders only when boolean changes):**
```tsx
function Sidebar() {
const isMobile = useMediaQuery('(max-width: 767px)')
return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```

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---
title: Use Functional setState Updates
impact: MEDIUM
impactDescription: prevents stale closures and unnecessary callback recreations
tags: react, hooks, useState, useCallback, callbacks, closures
---
## Use Functional setState Updates
When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.
**Incorrect (requires state as dependency):**
```tsx
function TodoList() {
const [items, setItems] = useState(initialItems)
// Callback must depend on items, recreated on every items change
const addItems = useCallback((newItems: Item[]) => {
setItems([...items, ...newItems])
}, [items]) // ❌ items dependency causes recreations
// Risk of stale closure if dependency is forgotten
const removeItem = useCallback((id: string) => {
setItems(items.filter(item => item.id !== id))
}, []) // ❌ Missing items dependency - will use stale items!
return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}
```
The first callback is recreated every time `items` changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial `items` value.
**Correct (stable callbacks, no stale closures):**
```tsx
function TodoList() {
const [items, setItems] = useState(initialItems)
// Stable callback, never recreated
const addItems = useCallback((newItems: Item[]) => {
setItems(curr => [...curr, ...newItems])
}, []) // ✅ No dependencies needed
// Always uses latest state, no stale closure risk
const removeItem = useCallback((id: string) => {
setItems(curr => curr.filter(item => item.id !== id))
}, []) // ✅ Safe and stable
return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />
}
```
**Benefits:**
1. **Stable callback references** - Callbacks don't need to be recreated when state changes
2. **No stale closures** - Always operates on the latest state value
3. **Fewer dependencies** - Simplifies dependency arrays and reduces memory leaks
4. **Prevents bugs** - Eliminates the most common source of React closure bugs
**When to use functional updates:**
- Any setState that depends on the current state value
- Inside useCallback/useMemo when state is needed
- Event handlers that reference state
- Async operations that update state
**When direct updates are fine:**
- Setting state to a static value: `setCount(0)`
- Setting state from props/arguments only: `setName(newName)`
- State doesn't depend on previous value
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.

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---
title: Use Lazy State Initialization
impact: MEDIUM
impactDescription: wasted computation on every render
tags: react, hooks, useState, performance, initialization
---
## Use Lazy State Initialization
Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.
**Incorrect (runs on every render):**
```tsx
function FilteredList({ items }: { items: Item[] }) {
// buildSearchIndex() runs on EVERY render, even after initialization
const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
const [query, setQuery] = useState('')
// When query changes, buildSearchIndex runs again unnecessarily
return <SearchResults index={searchIndex} query={query} />
}
function UserProfile() {
// JSON.parse runs on every render
const [settings, setSettings] = useState(
JSON.parse(localStorage.getItem('settings') || '{}')
)
return <SettingsForm settings={settings} onChange={setSettings} />
}
```
**Correct (runs only once):**
```tsx
function FilteredList({ items }: { items: Item[] }) {
// buildSearchIndex() runs ONLY on initial render
const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items))
const [query, setQuery] = useState('')
return <SearchResults index={searchIndex} query={query} />
}
function UserProfile() {
// JSON.parse runs only on initial render
const [settings, setSettings] = useState(() => {
const stored = localStorage.getItem('settings')
return stored ? JSON.parse(stored) : {}
})
return <SettingsForm settings={settings} onChange={setSettings} />
}
```
Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.
For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary.

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---
title: Extract Default Non-primitive Parameter Value from Memoized Component to Constant
impact: MEDIUM
impactDescription: restores memoization by using a constant for default value
tags: rerender, memo, optimization
---
## Extract Default Non-primitive Parameter Value from Memoized Component to Constant
When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in `memo()`.
To address this issue, extract the default value into a constant.
**Incorrect (`onClick` has different values on every rerender):**
```tsx
const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) {
// ...
})
// Used without optional onClick
<UserAvatar />
```
**Correct (stable default value):**
```tsx
const NOOP = () => {};
const UserAvatar = memo(function UserAvatar({ onClick = NOOP }: { onClick?: () => void }) {
// ...
})
// Used without optional onClick
<UserAvatar />
```

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---
title: Extract to Memoized Components
impact: MEDIUM
impactDescription: enables early returns
tags: rerender, memo, useMemo, optimization
---
## Extract to Memoized Components
Extract expensive work into memoized components to enable early returns before computation.
**Incorrect (computes avatar even when loading):**
```tsx
function Profile({ user, loading }: Props) {
const avatar = useMemo(() => {
const id = computeAvatarId(user)
return <Avatar id={id} />
}, [user])
if (loading) return <Skeleton />
return <div>{avatar}</div>
}
```
**Correct (skips computation when loading):**
```tsx
const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
const id = useMemo(() => computeAvatarId(user), [user])
return <Avatar id={id} />
})
function Profile({ user, loading }: Props) {
if (loading) return <Skeleton />
return (
<div>
<UserAvatar user={user} />
</div>
)
}
```
**Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders.

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---
title: Put Interaction Logic in Event Handlers
impact: MEDIUM
impactDescription: avoids effect re-runs and duplicate side effects
tags: rerender, useEffect, events, side-effects, dependencies
---
## Put Interaction Logic in Event Handlers
If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.
**Incorrect (event modeled as state + effect):**
```tsx
function Form() {
const [submitted, setSubmitted] = useState(false)
const theme = useContext(ThemeContext)
useEffect(() => {
if (submitted) {
post('/api/register')
showToast('Registered', theme)
}
}, [submitted, theme])
return <button onClick={() => setSubmitted(true)}>Submit</button>
}
```
**Correct (do it in the handler):**
```tsx
function Form() {
const theme = useContext(ThemeContext)
function handleSubmit() {
post('/api/register')
showToast('Registered', theme)
}
return <button onClick={handleSubmit}>Submit</button>
}
```
Reference: [Should this code move to an event handler?](https://react.dev/learn/removing-effect-dependencies#should-this-code-move-to-an-event-handler)

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---
title: Do not wrap a simple expression with a primitive result type in useMemo
impact: LOW-MEDIUM
impactDescription: wasted computation on every render
tags: rerender, useMemo, optimization
---
## Do not wrap a simple expression with a primitive result type in useMemo
When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in `useMemo`.
Calling `useMemo` and comparing hook dependencies may consume more resources than the expression itself.
**Incorrect:**
```tsx
function Header({ user, notifications }: Props) {
const isLoading = useMemo(() => {
return user.isLoading || notifications.isLoading
}, [user.isLoading, notifications.isLoading])
if (isLoading) return <Skeleton />
// return some markup
}
```
**Correct:**
```tsx
function Header({ user, notifications }: Props) {
const isLoading = user.isLoading || notifications.isLoading
if (isLoading) return <Skeleton />
// return some markup
}
```

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---
title: Use Transitions for Non-Urgent Updates
impact: MEDIUM
impactDescription: maintains UI responsiveness
tags: rerender, transitions, startTransition, performance
---
## Use Transitions for Non-Urgent Updates
Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.
**Incorrect (blocks UI on every scroll):**
```tsx
function ScrollTracker() {
const [scrollY, setScrollY] = useState(0)
useEffect(() => {
const handler = () => setScrollY(window.scrollY)
window.addEventListener('scroll', handler, { passive: true })
return () => window.removeEventListener('scroll', handler)
}, [])
}
```
**Correct (non-blocking updates):**
```tsx
import { startTransition } from 'react'
function ScrollTracker() {
const [scrollY, setScrollY] = useState(0)
useEffect(() => {
const handler = () => {
startTransition(() => setScrollY(window.scrollY))
}
window.addEventListener('scroll', handler, { passive: true })
return () => window.removeEventListener('scroll', handler)
}, [])
}
```

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---
title: Use useRef for Transient Values
impact: MEDIUM
impactDescription: avoids unnecessary re-renders on frequent updates
tags: rerender, useref, state, performance
---
## Use useRef for Transient Values
When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in `useRef` instead of `useState`. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.
**Incorrect (renders every update):**
```tsx
function Tracker() {
const [lastX, setLastX] = useState(0)
useEffect(() => {
const onMove = (e: MouseEvent) => setLastX(e.clientX)
window.addEventListener('mousemove', onMove)
return () => window.removeEventListener('mousemove', onMove)
}, [])
return (
<div
style={{
position: 'fixed',
top: 0,
left: lastX,
width: 8,
height: 8,
background: 'black',
}}
/>
)
}
```
**Correct (no re-render for tracking):**
```tsx
function Tracker() {
const lastXRef = useRef(0)
const dotRef = useRef<HTMLDivElement>(null)
useEffect(() => {
const onMove = (e: MouseEvent) => {
lastXRef.current = e.clientX
const node = dotRef.current
if (node) {
node.style.transform = `translateX(${e.clientX}px)`
}
}
window.addEventListener('mousemove', onMove)
return () => window.removeEventListener('mousemove', onMove)
}, [])
return (
<div
ref={dotRef}
style={{
position: 'fixed',
top: 0,
left: 0,
width: 8,
height: 8,
background: 'black',
transform: 'translateX(0px)',
}}
/>
)
}
```

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---
title: Use after() for Non-Blocking Operations
impact: MEDIUM
impactDescription: faster response times
tags: server, async, logging, analytics, side-effects
---
## Use after() for Non-Blocking Operations
Use Next.js's `after()` to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response.
**Incorrect (blocks response):**
```tsx
import { logUserAction } from '@/app/utils'
export async function POST(request: Request) {
// Perform mutation
await updateDatabase(request)
// Logging blocks the response
const userAgent = request.headers.get('user-agent') || 'unknown'
await logUserAction({ userAgent })
return new Response(JSON.stringify({ status: 'success' }), {
status: 200,
headers: { 'Content-Type': 'application/json' }
})
}
```
**Correct (non-blocking):**
```tsx
import { after } from 'next/server'
import { headers, cookies } from 'next/headers'
import { logUserAction } from '@/app/utils'
export async function POST(request: Request) {
// Perform mutation
await updateDatabase(request)
// Log after response is sent
after(async () => {
const userAgent = (await headers()).get('user-agent') || 'unknown'
const sessionCookie = (await cookies()).get('session-id')?.value || 'anonymous'
logUserAction({ sessionCookie, userAgent })
})
return new Response(JSON.stringify({ status: 'success' }), {
status: 200,
headers: { 'Content-Type': 'application/json' }
})
}
```
The response is sent immediately while logging happens in the background.
**Common use cases:**
- Analytics tracking
- Audit logging
- Sending notifications
- Cache invalidation
- Cleanup tasks
**Important notes:**
- `after()` runs even if the response fails or redirects
- Works in Server Actions, Route Handlers, and Server Components
Reference: [https://nextjs.org/docs/app/api-reference/functions/after](https://nextjs.org/docs/app/api-reference/functions/after)

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---
title: Authenticate Server Actions Like API Routes
impact: CRITICAL
impactDescription: prevents unauthorized access to server mutations
tags: server, server-actions, authentication, security, authorization
---
## Authenticate Server Actions Like API Routes
**Impact: CRITICAL (prevents unauthorized access to server mutations)**
Server Actions (functions with `"use server"`) are exposed as public endpoints, just like API routes. Always verify authentication and authorization **inside** each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly.
Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation."
**Incorrect (no authentication check):**
```typescript
'use server'
export async function deleteUser(userId: string) {
// Anyone can call this! No auth check
await db.user.delete({ where: { id: userId } })
return { success: true }
}
```
**Correct (authentication inside the action):**
```typescript
'use server'
import { verifySession } from '@/lib/auth'
import { unauthorized } from '@/lib/errors'
export async function deleteUser(userId: string) {
// Always check auth inside the action
const session = await verifySession()
if (!session) {
throw unauthorized('Must be logged in')
}
// Check authorization too
if (session.user.role !== 'admin' && session.user.id !== userId) {
throw unauthorized('Cannot delete other users')
}
await db.user.delete({ where: { id: userId } })
return { success: true }
}
```
**With input validation:**
```typescript
'use server'
import { verifySession } from '@/lib/auth'
import { z } from 'zod'
const updateProfileSchema = z.object({
userId: z.string().uuid(),
name: z.string().min(1).max(100),
email: z.string().email()
})
export async function updateProfile(data: unknown) {
// Validate input first
const validated = updateProfileSchema.parse(data)
// Then authenticate
const session = await verifySession()
if (!session) {
throw new Error('Unauthorized')
}
// Then authorize
if (session.user.id !== validated.userId) {
throw new Error('Can only update own profile')
}
// Finally perform the mutation
await db.user.update({
where: { id: validated.userId },
data: {
name: validated.name,
email: validated.email
}
})
return { success: true }
}
```
Reference: [https://nextjs.org/docs/app/guides/authentication](https://nextjs.org/docs/app/guides/authentication)

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---
title: Cross-Request LRU Caching
impact: HIGH
impactDescription: caches across requests
tags: server, cache, lru, cross-request
---
## Cross-Request LRU Caching
`React.cache()` only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache.
**Implementation:**
```typescript
import { LRUCache } from 'lru-cache'
const cache = new LRUCache<string, any>({
max: 1000,
ttl: 5 * 60 * 1000 // 5 minutes
})
export async function getUser(id: string) {
const cached = cache.get(id)
if (cached) return cached
const user = await db.user.findUnique({ where: { id } })
cache.set(id, user)
return user
}
// Request 1: DB query, result cached
// Request 2: cache hit, no DB query
```
Use when sequential user actions hit multiple endpoints needing the same data within seconds.
**With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis.
**In traditional serverless:** Each invocation runs in isolation, so consider Redis for cross-process caching.
Reference: [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache)

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---
title: Per-Request Deduplication with React.cache()
impact: MEDIUM
impactDescription: deduplicates within request
tags: server, cache, react-cache, deduplication
---
## Per-Request Deduplication with React.cache()
Use `React.cache()` for server-side request deduplication. Authentication and database queries benefit most.
**Usage:**
```typescript
import { cache } from 'react'
export const getCurrentUser = cache(async () => {
const session = await auth()
if (!session?.user?.id) return null
return await db.user.findUnique({
where: { id: session.user.id }
})
})
```
Within a single request, multiple calls to `getCurrentUser()` execute the query only once.
**Avoid inline objects as arguments:**
`React.cache()` uses shallow equality (`Object.is`) to determine cache hits. Inline objects create new references each call, preventing cache hits.
**Incorrect (always cache miss):**
```typescript
const getUser = cache(async (params: { uid: number }) => {
return await db.user.findUnique({ where: { id: params.uid } })
})
// Each call creates new object, never hits cache
getUser({ uid: 1 })
getUser({ uid: 1 }) // Cache miss, runs query again
```
**Correct (cache hit):**
```typescript
const getUser = cache(async (uid: number) => {
return await db.user.findUnique({ where: { id: uid } })
})
// Primitive args use value equality
getUser(1)
getUser(1) // Cache hit, returns cached result
```
If you must pass objects, pass the same reference:
```typescript
const params = { uid: 1 }
getUser(params) // Query runs
getUser(params) // Cache hit (same reference)
```
**Next.js-Specific Note:**
In Next.js, the `fetch` API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need `React.cache()` for `fetch` calls. However, `React.cache()` is still essential for other async tasks:
- Database queries (Prisma, Drizzle, etc.)
- Heavy computations
- Authentication checks
- File system operations
- Any non-fetch async work
Use `React.cache()` to deduplicate these operations across your component tree.
Reference: [React.cache documentation](https://react.dev/reference/react/cache)

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---
title: Avoid Duplicate Serialization in RSC Props
impact: LOW
impactDescription: reduces network payload by avoiding duplicate serialization
tags: server, rsc, serialization, props, client-components
---
## Avoid Duplicate Serialization in RSC Props
**Impact: LOW (reduces network payload by avoiding duplicate serialization)**
RSC→client serialization deduplicates by object reference, not value. Same reference = serialized once; new reference = serialized again. Do transformations (`.toSorted()`, `.filter()`, `.map()`) in client, not server.
**Incorrect (duplicates array):**
```tsx
// RSC: sends 6 strings (2 arrays × 3 items)
<ClientList usernames={usernames} usernamesOrdered={usernames.toSorted()} />
```
**Correct (sends 3 strings):**
```tsx
// RSC: send once
<ClientList usernames={usernames} />
// Client: transform there
'use client'
const sorted = useMemo(() => [...usernames].sort(), [usernames])
```
**Nested deduplication behavior:**
Deduplication works recursively. Impact varies by data type:
- `string[]`, `number[]`, `boolean[]`: **HIGH impact** - array + all primitives fully duplicated
- `object[]`: **LOW impact** - array duplicated, but nested objects deduplicated by reference
```tsx
// string[] - duplicates everything
usernames={['a','b']} sorted={usernames.toSorted()} // sends 4 strings
// object[] - duplicates array structure only
users={[{id:1},{id:2}]} sorted={users.toSorted()} // sends 2 arrays + 2 unique objects (not 4)
```
**Operations breaking deduplication (create new references):**
- Arrays: `.toSorted()`, `.filter()`, `.map()`, `.slice()`, `[...arr]`
- Objects: `{...obj}`, `Object.assign()`, `structuredClone()`, `JSON.parse(JSON.stringify())`
**More examples:**
```tsx
// ❌ Bad
<C users={users} active={users.filter(u => u.active)} />
<C product={product} productName={product.name} />
// ✅ Good
<C users={users} />
<C product={product} />
// Do filtering/destructuring in client
```
**Exception:** Pass derived data when transformation is expensive or client doesn't need original.

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---
title: Hoist Static I/O to Module Level
impact: HIGH
impactDescription: avoids repeated file/network I/O per request
tags: server, io, performance, next.js, route-handlers, og-image
---
## Hoist Static I/O to Module Level
**Impact: HIGH (avoids repeated file/network I/O per request)**
When loading static assets (fonts, logos, images, config files) in route handlers or server functions, hoist the I/O operation to module level. Module-level code runs once when the module is first imported, not on every request. This eliminates redundant file system reads or network fetches that would otherwise run on every invocation.
**Incorrect: reads font file on every request**
```typescript
// app/api/og/route.tsx
import { ImageResponse } from 'next/og'
export async function GET(request: Request) {
// Runs on EVERY request - expensive!
const fontData = await fetch(
new URL('./fonts/Inter.ttf', import.meta.url)
).then(res => res.arrayBuffer())
const logoData = await fetch(
new URL('./images/logo.png', import.meta.url)
).then(res => res.arrayBuffer())
return new ImageResponse(
<div style={{ fontFamily: 'Inter' }}>
<img src={logoData} />
Hello World
</div>,
{ fonts: [{ name: 'Inter', data: fontData }] }
)
}
```
**Correct: loads once at module initialization**
```typescript
// app/api/og/route.tsx
import { ImageResponse } from 'next/og'
// Module-level: runs ONCE when module is first imported
const fontData = fetch(
new URL('./fonts/Inter.ttf', import.meta.url)
).then(res => res.arrayBuffer())
const logoData = fetch(
new URL('./images/logo.png', import.meta.url)
).then(res => res.arrayBuffer())
export async function GET(request: Request) {
// Await the already-started promises
const [font, logo] = await Promise.all([fontData, logoData])
return new ImageResponse(
<div style={{ fontFamily: 'Inter' }}>
<img src={logo} />
Hello World
</div>,
{ fonts: [{ name: 'Inter', data: font }] }
)
}
```
**Alternative: synchronous file reads with Node.js fs**
```typescript
// app/api/og/route.tsx
import { ImageResponse } from 'next/og'
import { readFileSync } from 'fs'
import { join } from 'path'
// Synchronous read at module level - blocks only during module init
const fontData = readFileSync(
join(process.cwd(), 'public/fonts/Inter.ttf')
)
const logoData = readFileSync(
join(process.cwd(), 'public/images/logo.png')
)
export async function GET(request: Request) {
return new ImageResponse(
<div style={{ fontFamily: 'Inter' }}>
<img src={logoData} />
Hello World
</div>,
{ fonts: [{ name: 'Inter', data: fontData }] }
)
}
```
**General Node.js example: loading config or templates**
```typescript
// Incorrect: reads config on every call
export async function processRequest(data: Data) {
const config = JSON.parse(
await fs.readFile('./config.json', 'utf-8')
)
const template = await fs.readFile('./template.html', 'utf-8')
return render(template, data, config)
}
// Correct: loads once at module level
const configPromise = fs.readFile('./config.json', 'utf-8')
.then(JSON.parse)
const templatePromise = fs.readFile('./template.html', 'utf-8')
export async function processRequest(data: Data) {
const [config, template] = await Promise.all([
configPromise,
templatePromise
])
return render(template, data, config)
}
```
**When to use this pattern:**
- Loading fonts for OG image generation
- Loading static logos, icons, or watermarks
- Reading configuration files that don't change at runtime
- Loading email templates or other static templates
- Any static asset that's the same across all requests
**When NOT to use this pattern:**
- Assets that vary per request or user
- Files that may change during runtime (use caching with TTL instead)
- Large files that would consume too much memory if kept loaded
- Sensitive data that shouldn't persist in memory
**With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** Module-level caching is especially effective because multiple concurrent requests share the same function instance. The static assets stay loaded in memory across requests without cold start penalties.
**In traditional serverless:** Each cold start re-executes module-level code, but subsequent warm invocations reuse the loaded assets until the instance is recycled.

83
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---
title: Parallel Data Fetching with Component Composition
impact: CRITICAL
impactDescription: eliminates server-side waterfalls
tags: server, rsc, parallel-fetching, composition
---
## Parallel Data Fetching with Component Composition
React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching.
**Incorrect (Sidebar waits for Page's fetch to complete):**
```tsx
export default async function Page() {
const header = await fetchHeader()
return (
<div>
<div>{header}</div>
<Sidebar />
</div>
)
}
async function Sidebar() {
const items = await fetchSidebarItems()
return <nav>{items.map(renderItem)}</nav>
}
```
**Correct (both fetch simultaneously):**
```tsx
async function Header() {
const data = await fetchHeader()
return <div>{data}</div>
}
async function Sidebar() {
const items = await fetchSidebarItems()
return <nav>{items.map(renderItem)}</nav>
}
export default function Page() {
return (
<div>
<Header />
<Sidebar />
</div>
)
}
```
**Alternative with children prop:**
```tsx
async function Header() {
const data = await fetchHeader()
return <div>{data}</div>
}
async function Sidebar() {
const items = await fetchSidebarItems()
return <nav>{items.map(renderItem)}</nav>
}
function Layout({ children }: { children: ReactNode }) {
return (
<div>
<Header />
{children}
</div>
)
}
export default function Page() {
return (
<Layout>
<Sidebar />
</Layout>
)
}
```

38
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@ -0,0 +1,38 @@
---
title: Minimize Serialization at RSC Boundaries
impact: HIGH
impactDescription: reduces data transfer size
tags: server, rsc, serialization, props
---
## Minimize Serialization at RSC Boundaries
The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses.
**Incorrect (serializes all 50 fields):**
```tsx
async function Page() {
const user = await fetchUser() // 50 fields
return <Profile user={user} />
}
'use client'
function Profile({ user }: { user: User }) {
return <div>{user.name}</div> // uses 1 field
}
```
**Correct (serializes only 1 field):**
```tsx
async function Page() {
const user = await fetchUser()
return <Profile name={user.name} />
}
'use client'
function Profile({ name }: { name: string }) {
return <div>{name}</div>
}
```

39
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@ -0,0 +1,39 @@
---
name: web-design-guidelines
description: Review UI code for Web Interface Guidelines compliance. Use when asked to "review my UI", "check accessibility", "audit design", "review UX", or "check my site against best practices".
metadata:
author: vercel
version: "1.0.0"
argument-hint: <file-or-pattern>
---
# Web Interface Guidelines
Review files for compliance with Web Interface Guidelines.
## How It Works
1. Fetch the latest guidelines from the source URL below
2. Read the specified files (or prompt user for files/pattern)
3. Check against all rules in the fetched guidelines
4. Output findings in the terse `file:line` format
## Guidelines Source
Fetch fresh guidelines before each review:
```
https://raw.githubusercontent.com/vercel-labs/web-interface-guidelines/main/command.md
```
Use WebFetch to retrieve the latest rules. The fetched content contains all the rules and output format instructions.
## Usage
When a user provides a file or pattern argument:
1. Fetch guidelines from the source URL above
2. Read the specified files
3. Apply all rules from the fetched guidelines
4. Output findings using the format specified in the guidelines
If no files specified, ask the user which files to review.

20
skills-lock.json Normal file
View file

@ -0,0 +1,20 @@
{
"version": 1,
"skills": {
"shadcn": {
"source": "shadcn/ui",
"sourceType": "github",
"computedHash": "b1cc1defe03f0e8f78652b7d20e4d4e6421babc2b45835d4f361afe684dc47a2"
},
"vercel-react-best-practices": {
"source": "vercel-labs/agent-skills",
"sourceType": "github",
"computedHash": "3ff06f0712886f7c2660d6746856c22d3127bd6568ddde29ddad8b72dbaaa226"
},
"web-design-guidelines": {
"source": "vercel-labs/agent-skills",
"sourceType": "github",
"computedHash": "d8e7d3afe37dcc8a97b99ffb5afdb4d0919ae0092ea8b68f44eb201f035e33ac"
}
}
}