Master Postman: Advanced Environment Variables and Pre‑request Scripts for API Testing

Environment Variables

Postman allows you to define global and environment‑specific variables such as host and port, which can be referenced in request URLs using the {{variable}} syntax. You can create multiple environments (e.g., local, test) and switch between them, with the active environment overriding globals when names clash.

Pre‑request Script

Running JavaScript

Pre‑request scripts execute JavaScript before a request is sent. For example, you can set variables programmatically:

pm.globals.set("key1","value1");
pm.environment.set("key2","value2");

You can also delete variables:

pm.globals.unset("key1");
pm.environment.unset("key2");

Sending a GET request from a script

Use pm.sendRequest to issue an auxiliary GET request and log the result to the console:

pm.sendRequest({url: "http://localhost/api", method: "GET"}, function (err, res) { console.log(res); });

Sending a POST request for token acquisition

When an API requires authentication, you can fetch a JWT token in a pre‑request script, store it in a collection variable, and reuse it in subsequent requests:

pm.sendRequest({url: "http://localhost/login", method: "POST", body: {mode: "raw", raw: JSON.stringify({user:"admin",pass:"123"})}}, function (err, res) { var token = res.json().data.token; pm.collectionVariables.set("TOKEN", token); });

Later, include the token in request headers using {{TOKEN}}.

Tests

Test scripts run after a request completes. They can also store values, such as caching a JWT token to avoid repeated logins:

var token = pm.response.json().data.token; pm.collectionVariables.set("TOKEN", token);

The execution order is: collection‑level pre‑request scripts first, then folder‑level, then request‑level; after the request, the same hierarchy applies to test scripts.