Add JavaScript version to render the hash digest.

URL-signing.md

@@ -89,4 +89,15 @@ import base64
 key = 'secret key'
 data = 'https://octodex.github.com/images/codercat.jpg'
 print base64.urlsafe_b64encode(hmac.new(key, msg=data, digestmod=hashlib.sha256).digest()) 
+```
+
+### JavaScript
+
+```javascript
+import crypto from 'crypto';
+import URLSafeBase64 from 'urlsafe-base64';
+
+let key = 'secret key';
+let data = 'https://octodex.github.com/images/codercat.jpg';
+console.log(URLSafeBase64.encode(crypto.createHmac('sha256', key).update(data).digest()));
 ```