Reusable test scripts in Postman - Build an Automation Script

Verify user login API with reusable test scripts

Preconditions (3)

Step 1: Create a new Postman collection named 'User Authentication'

Step 2: Add a new request named 'Login' with POST method to https://api.example.com/login

Step 3: In the request body, add JSON with fields 'username' and 'password'

Step 4: Write a reusable test script in the collection's Tests tab to verify response status is 200

Step 5: Also verify the response JSON contains a 'token' field

Step 6: Save the reusable test script in the collection's Pre-request Scripts or Tests section

Step 7: Run the 'Login' request with valid credentials

Step 8: Observe that the reusable test script runs and validates the response

✅ Expected Result: The reusable test script runs after the 'Login' request and asserts the response status is 200 and the response contains a 'token' field, passing the test.

Automation Requirements - Postman

Assertions Needed:

Response status code is 200

Response JSON contains 'token' field

Best Practices:

Use collection-level test scripts for reuse

Avoid duplicating test code in individual requests

Use pm.expect assertions for clear validation

Keep test scripts simple and readable

Automated Solution

Postman

/* Collection-level Tests script in Postman */
pm.test('Response status is 200', () => {
    pm.response.to.have.status(200);
});
pm.test('Response has token field', () => {
    const jsonData = pm.response.json();
    pm.expect(jsonData).to.have.property('token');
});

This script is placed at the collection level so it runs after every request in the collection.

First, it checks if the response status code is 200, which means success.

Second, it parses the response JSON and checks if there is a 'token' property, which indicates a successful login.

Using collection-level scripts avoids repeating the same tests in every request, making maintenance easier.

Common Mistakes - 3 Pitfalls

Writing test scripts only in individual requests instead of collection level

Not checking response status before accessing JSON fields

Using console.log instead of pm.expect for assertions

Bonus Challenge

Now add data-driven testing by running the login request with 3 different sets of user credentials using Postman environment variables or data files.

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