Defining mock responses in Postman - Build an Automation Script

Create and verify a mock response for a GET user API

Preconditions (2)

Step 1: Open the Postman app and select the collection containing GET /user

Step 2: Click on the 'Mocks' tab in the collection view

Step 3: Click 'Create a mock server'

Step 4: Set the mock server name to 'User API Mock'

Step 5: Select the GET /user request to mock

Step 6: Define the mock response with status 200 and JSON body: {"id":1,"name":"John Doe"}

Step 7: Save the mock server

Step 8: Send a GET request to the mock server URL for /user

Step 9: Verify the response status is 200

Step 10: Verify the response body matches {"id":1,"name":"John Doe"}

✅ Expected Result: The mock server returns the defined JSON response with status 200 when GET /user is called

Automation Requirements - Postman test scripts

Assertions Needed:

Response status code is 200

Response body JSON matches the mock data

Best Practices:

Use Postman built-in pm.test and pm.response APIs for assertions

Keep mock response JSON simple and clear

Use environment variables for mock server URL

Validate both status code and response body

Automated Solution

Postman

pm.test('Status code is 200', () => {
    pm.response.to.have.status(200);
});

pm.test('Response body matches mock data', () => {
    const jsonData = pm.response.json();
    pm.expect(jsonData).to.eql({ id: 1, name: 'John Doe' });
});

The first test checks that the response status code is exactly 200, which means success.

The second test parses the response body as JSON and compares it to the expected mock object with id 1 and name 'John Doe'.

Using pm.test groups assertions with clear names, making test reports easy to understand.

This script runs automatically after sending the request to the mock server, verifying the mock response is correct.

Common Mistakes - 3 Pitfalls

Not verifying the response status code

Comparing response body as a string instead of JSON object

Hardcoding mock server URL in tests

Bonus Challenge

Now add data-driven testing by defining three different mock responses for GET /user with different user data and verify each response.

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