Update prerequest-scripts.md

docs/docs/features/prerequest-scripts.md

@@ -51,4 +51,33 @@ const sessid = altair.helpers.getCookie('sessid');
 ```js
 const res = await altair.helpers.request('GET', 'https://api.agify.io/?name=michael');
 // res => {"name":"michael","age":60,"count":41938}
-```
+```
+
+### Persisting data between requests
+
+Since altair pretty much lives in a browser environment, it does support the `LocalStorage` feature. This is useful when you need an authentication token before each request but only requesting the token when your authentication expired.
+
+Assuming you have this header in the `Set Headers window`:
+
+| Key           | Value                |
+| ------------- | -------------------- |
+| Authorization | Bearer {{token_env}} |
+
+Below is a working example of persisting data between requests (token, token_expiry):
+
+```js
+const nowInSeconds = () => Date.now() / 1000;
+const tokenExpiry = localStorage.getItem("token_expiry") || 0;
+
+if (nowInSeconds() >= tokenExpiry) {
+  const res = await altair.helpers.request('POST', 'https://auth.example.com', { /* auth payload */ });
+  // res => {"token":"abcd","expiry":3600}
+
+  localStorage.setItem("token", res.token);
+  localStorage.setItem("token_expiry", nowInSeconds() + res.expiry);
+}
+
+const token = localStorage.getItem("token");
+
+altair.helpers.setEnvironment('token_env', token);
+```