What is the difference between Pre-request Scripts and Tests in Postman?

Answer

Pre-request Scripts vs Tests in Postman

Both are JavaScript tabs in Postman, but they run at different times and serve different purposes.

✦

Pre-request Scripts

Run BEFORE the request is sent. Used for setup and preparation.

Use Cases

✓Generate dynamic values (timestamps, random data)
✓Set authentication tokens
✓Calculate checksums or signatures
✓Chain requests by using data from a previous response

Examples

JavaScript

// Generate a random email
const randomEmail = `user_${Date.now()}@test.com`;
pm.environment.set("random_email", randomEmail);

// Get current timestamp
pm.environment.set("timestamp", new Date().toISOString());

// Fetch a token before the actual request (via pm.sendRequest)
pm.sendRequest({
    url: pm.environment.get("base_url") + "/auth/login",
    method: "POST",
    header: { "Content-Type": "application/json" },
    body: {
        mode: "raw",
        raw: JSON.stringify({ username: "admin", password: "secret" })
    }
}, function (err, res) {
    pm.environment.set("auth_token", res.json().token);
});

✦

Tests (Post-response Scripts)

Run AFTER the response is received. Used for validation.

Use Cases

✓Assert status codes
✓Validate response body fields
✓Check response headers
✓Save values from response for next request
✓Log debugging info

Examples

JavaScript

pm.test("Status is 201", function () {
    pm.response.to.have.status(201);
});

const json = pm.response.json();
pm.test("Name matches", function () {
    pm.expect(json.name).to.eql(pm.environment.get("random_name"));
});

// Save for next request
pm.environment.set("created_id", json.id);

✦

Summary

Feature	Pre-request Script	Tests (Post-response)
Runs	Before request	After response
Purpose	Setup / Preparation	Validation / Assertions
Access to response	No	Yes
Common use	Token generation, dynamic data	Status code checks, body validation