Writing Test Scripts in Postman with JavaScript

Writing Test Scripts in Postman with JavaScript

Writing automated tests is what separates professional Postman API testing from simply using Postman as a glorified browser. The Tests tab in every request lets you write JavaScript assertions that automatically validate responses — turning manual verification into automated quality assurance. Every API test suite should include these scripts.

The pm Object

Postman provides a globally available pm object in all scripts with methods for everything you need:

• pm.test(name, fn): Define a named test assertion.
• pm.expect(value): Chai.js assertion library for expressive assertions.
• pm.response: Access the response object — status, body, headers, timing.
• pm.environment: Get and set environment variables.
• pm.globals: Get and set global variables.
• pm.request: Access the request that was sent.

Basic Test Structure

// Test 1: Status code check
pm.test("Status code is 200", function () {
  pm.response.to.have.status(200);
});

// Test 2: Response time check
pm.test("Response time is under 500ms", function () {
  pm.expect(pm.response.responseTime).to.be.below(500);
});

// Test 3: Response body is JSON
pm.test("Response is JSON", function () {
  pm.response.to.be.json;
});

// Test 4: Parse and inspect body
const jsonData = pm.response.json();

pm.test("User has required fields", function () {
  pm.expect(jsonData).to.have.property("id");
  pm.expect(jsonData).to.have.property("name");
  pm.expect(jsonData).to.have.property("email");
});

// Test 5: Data type validation
pm.test("ID is a number", function () {
  pm.expect(jsonData.id).to.be.a("number");
});

// Test 6: Content-Type header
pm.test("Content-Type is application/json", function () {
  pm.expect(pm.response.headers.get("Content-Type")).to.include("application/json");
});

Testing Arrays and Nested Objects

const data = pm.response.json();

// Array tests
pm.test("Response is an array", function () {
  pm.expect(data).to.be.an("array");
});

pm.test("Array has at least one item", function () {
  pm.expect(data.length).to.be.above(0);
});

pm.test("First item has correct structure", function () {
  const first = data[0];
  pm.expect(first).to.have.all.keys("id", "title", "body", "userId");
});

// Nested object
pm.test("Nested address has city", function () {
  pm.expect(data[0].address.city).to.be.a("string");
});

Testing Error Responses

// For a 404 response test:
pm.test("Returns 404 for missing resource", function () {
  pm.response.to.have.status(404);
});

pm.test("Error message is descriptive", function () {
  const error = pm.response.json();
  pm.expect(error).to.have.property("message");
  pm.expect(error.message).to.be.a("string").and.to.not.be.empty;
});

Saving Values from Responses

// After POST /users - save new user's ID for subsequent requests
const newUser = pm.response.json();
pm.environment.set("created_user_id", newUser.id);
pm.environment.set("created_user_email", newUser.email);
console.log("Created user ID:", newUser.id);

Using Snippets

Postman provides ready-made test snippets in the right sidebar of the Tests tab. Click any snippet to insert it — this is the fastest way to add common tests without typing from scratch. Available snippets include: Status code is 200, Response body contains string, Response time is less than 200ms, and many others.

Key Takeaways

• Every production API test should check: status code, response time, content type, and body structure.
• Use pm.response.json() to parse the body, then pm.expect() to assert values.
• Save response values to environment variables to chain tests across multiple requests.
• Use Postman's built-in snippets to quickly add common assertions without writing from scratch.