Mock vs stub comparison in Postman - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference between mock and stub in API testing

Which statement best describes the difference between a mock and a stub in API testing using Postman?

A A stub provides predefined responses to API calls without behavior verification, while a mock also verifies that the API calls were made as expected.

B A mock only provides static responses, whereas a stub dynamically changes responses based on request data.

C A stub verifies the sequence of API calls, but a mock only returns fixed responses without verification.

D Mocks and stubs are identical; both only simulate API responses without any verification.

💡 Hint

Think about whether the tool checks if the API was called correctly or just returns data.

Stubs return fixed responses to API calls but do not check if the calls happened as expected. Mocks do both: they return responses and verify the calls.

❓ Predict Output

intermediate

Postman mock server response behavior

Given this Postman mock server setup that returns a fixed JSON response, what will be the response body when a GET request is sent?

Postman

{
  "message": "Hello from mock",
  "status": 200
}

A

{
  "error": "Not found",
  "status": 404
}

B

{
  "message": "Hello from stub",
  "status": 200
}

C

{
  "message": "Hello from mock",
  "status": 200
}

D Empty response with status 204

💡 Hint

The mock server returns the predefined response exactly as configured.

The mock server always returns the fixed JSON response set up in Postman, regardless of the request details.

❓ assertion

advanced

Valid assertion for verifying stub response in Postman test script

Which Postman test script assertion correctly verifies that the stubbed API response contains a 'userId' field with value 5?

Postman

pm.test("Check userId is 5", () => {
  // assertion here
});

A pm.expect(pm.response.json().userId).to.eql(5);

B pm.expect(pm.response.userId).to.equal(5);

C pm.expect(pm.response.json.userId).toBe(5);

D pm.expect(pm.response.body.userId).to.eql(5);

💡 Hint

Remember to parse JSON and use the correct assertion method in Postman.

pm.response.json() parses the response body as JSON. The method to check equality in Postman is 'to.eql()'.

🔧 Debug

advanced

Identify the error in Postman mock test script

What error will this Postman test script produce when run against a mock server response?

Postman

pm.test("Response has status 200", () => {
  pm.expect(pm.response.status).to.equal(200);
});

A SyntaxError: Unexpected token '.'

B AssertionError: expected 200 to equal 200

C No error, test passes successfully

D AssertionError: expected 'OK' to equal 200

💡 Hint

Check how to access the response status code in Postman scripts.

In Postman, pm.response.status returns the status text (e.g., 'OK'), not the numeric code. The correct property is pm.response.code for status code.

❓ framework

expert

Best practice for combining mocks and stubs in Postman API testing

Which approach best uses both mocks and stubs in Postman to test an API that depends on an external service?

A Use only mocks for both returning data and verifying calls, ignoring stubs completely.

B Use a stub to return fixed data for the external service and a mock to verify the API calls to that service during integration tests.

C Use stubs to verify API call sequences and mocks to return dynamic responses based on request data.

D Use mocks to simulate the external service responses without any verification, and stubs to check call correctness.

💡 Hint

Think about separating response simulation and call verification roles.

Stubs provide fixed responses to simulate external services. Mocks add verification to ensure the API calls happen as expected. Combining both improves test reliability.