Writing API Tests with Postman (With Examples)

🚀 Why API Testing Matters

API testing is a must-have skill for modern QA professionals. It helps you test the logic and functionality behind the user interface — often before the UI is even built.

Postman is one of the easiest tools to start with. Whether you’re a manual tester or learning automation, Postman helps you send requests, validate responses, and even write test scripts — all without needing to write full code.

🔧 What Is Postman?

Postman is a free API client that lets you:

Send requests to REST APIs
Inspect responses (status, body, headers)
Write basic tests using JavaScript
Group requests into collections and environments

✅ Step-by-Step: Writing Your First API Test in Postman

Let’s go through an example using a public API:
🔗 https://jsonplaceholder.typicode.com

Step 1: Set Up Postman

Download Postman from postman.com
Open the app and create a free account

Step 2: Make a Simple GET Request

Let’s get a list of fake posts from JSONPlaceholder:

URL:

arduinoCopyEdithttps://jsonplaceholder.typicode.com/posts

Method:
GET

Click Send — you’ll see a JSON response containing posts.

Step 3: Write a Test to Validate the Response

Scroll down to the Tests tab and paste this code:

javascriptCopyEditpm.test("Status code is 200", function () {
    pm.response.to.have.status(200);
});

pm.test("Response has at least one post", function () {
    var jsonData = pm.response.json();
    pm.expect(jsonData.length).to.be.above(0);
});

🔍 What this does:

Asserts that the status code is 200
Checks that the response contains at least one post

Step 4: Test a POST Request

Let’s send a POST request to create a new post.

URL:

arduinoCopyEdithttps://jsonplaceholder.typicode.com/posts

Method:
POST

Body → raw → JSON:

jsonCopyEdit{
  "title": "QA Playbook",
  "body": "Learning API testing with Postman",
  "userId": 1
}

Tests Tab:

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

pm.test("Post contains correct title", function () {
    var jsonData = pm.response.json();
    pm.expect(jsonData.title).to.eql("QA Playbook");
});

✅ This validates the response from a simulated post creation.

🧪 More Test Snippets for Beginners

✅ Check Response Time

javascriptCopyEditpm.test("Response time is less than 500ms", function () {
    pm.expect(pm.response.responseTime).to.be.below(500);
});

✅ Check Content Type Header

javascriptCopyEditpm.test("Content-Type is application/json", function () {
    pm.response.to.have.header("Content-Type");
});

✅ Validate a Property in the Response

javascriptCopyEditpm.test("User ID is 1", function () {
    var jsonData = pm.response.json();
    pm.expect(jsonData.userId).to.eql(1);
});

💡 Bonus Tips

Use Environment Variables for dynamic data (e.g., {{base_url}})
Save your requests to Collections to reuse them
Export your tests and share with your team
Integrate with Newman CLI to run Postman tests from the command line

📁 Common Use Cases for Postman in QA

Use Case	Postman Feature
Validate API response format	`Tests` with `pm.response.json()`
Check auth/login APIs	Use `Authorization` tab
Automate test runs	Use Newman CLI + scripts
Mock APIs for frontend devs	Postman’s Mock Server feature

🎯 Final Thoughts

API testing doesn’t have to be overwhelming. Postman makes it simple to:

Explore and understand how APIs work
Validate backend logic without waiting for the UI
Build confidence in testing more than just interfaces