add javascript example for truncated signature (#163)

examples/signature-truncated.mjs

@@ -0,0 +1,21 @@
+import { createHmac } from 'node:crypto';
+
+const KEY = '943b421c9eb07c830af81030552c86009268de4e532ba2ee2eab8247c6da0881'
+const SALT = '520f986b998545b4785e0defbc4f3c1203f22de2374a3d53cb7a7fe9fea309c5'
+const SIGNATURE_SIZE = 8 // should correspond to IMGPROXY_SIGNATURE_SIZE on the server. Default: 32
+
+const hexDecode = (hex) => Buffer.from(hex, 'hex')
+
+const sign = (salt, target, secret, size) => {
+  const hmac = createHmac('sha256', hexDecode(secret))
+  hmac.update(hexDecode(salt))
+  hmac.update(target)
+
+  return Buffer.from(hmac.digest().slice(0, size)).toString('base64url')
+}
+
+const path = "/rs:fit:300:300/plain/http://img.example.com/pretty/image.jpg"
+
+const signature = sign(SALT, path, KEY, SIGNATURE_SIZE)
+const result = `/${signature}${path}`
+console.log(result)