Fixture-based response mocking in Cypress - Test Execution Trace

Test Overview

This test uses a fixture file to mock the server response for a user list API call. It verifies that the mocked data is displayed correctly on the page.

Test Code - Cypress

Cypress

describe('User List with Fixture Mocking', () => {
  it('displays mocked user data from fixture', () => {
    cy.intercept('GET', '/api/users', { fixture: 'users.json' }).as('getUsers');
    cy.visit('/users');
    cy.wait('@getUsers');
    cy.get('[data-cy=user-name]').should('have.length', 3);
    cy.get('[data-cy=user-name]').first().should('contain.text', 'Alice');
  });
});

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Set up intercept to mock GET /api/users with fixture 'users.json' and alias it as 'getUsers'	No page loaded yet, intercept ready to catch API call	—	PASS
2	Visit the /users page in the browser	Browser loads the /users page, triggering API call to /api/users	—	PASS
3	Wait for the intercepted API call '@getUsers' to complete	API call intercepted and responded with fixture data	Verify the mocked response was used	PASS
4	Find all elements with data-cy attribute 'user-name' on the page	Page displays user names from mocked data	Check that there are exactly 3 user name elements	PASS
5	Check that the first user name element contains text 'Alice'	First user name element visible with text 'Alice'	Verify the first user name matches the fixture data	PASS

Failure Scenario

Failing Condition: The fixture file 'users.json' is missing or malformed, or the intercept does not match the API call

Execution Trace Quiz - 3 Questions

What does the intercept command do in this test?

A It clicks on the first user name element

B It waits for the page to load completely

C It replaces the real API response with data from the fixture file

D It verifies the number of user elements on the page

Key Result

Using fixture-based mocking helps isolate frontend tests from backend changes and makes tests faster and more reliable by controlling API responses.

Practice

1. What is the main purpose of using fixture-based response mocking in Cypress tests?

easy

A. To replace real server responses with fixed data files for consistent testing

B. To speed up the browser rendering process

C. To automatically generate test data from the server

D. To record user interactions for playback

Solution

1. Step 1: Understand fixture-based mocking concept

   Fixture-based mocking replaces actual server responses with predefined data files to control test conditions.

2. Step 2: Identify the main benefit

   This approach ensures tests run fast and reliably by avoiding real server calls and unpredictable data.

3. Final Answer:

   To replace real server responses with fixed data files for consistent testing -> Option A

4. Quick Check:

   Fixture mocking = fixed data for reliable tests [OK]

Hint: Fixture mocking means fixed data replaces server calls [OK]

Common Mistakes:

• Confusing fixture mocking with UI rendering speed
• Thinking fixture mocking generates data automatically
• Assuming fixture mocking records user actions

2. Which of the following is the correct syntax to mock a GET request to '/api/users' using a fixture named 'users.json' in Cypress?

easy

A. cy.mock('GET', '/api/users', 'users.json')

B. cy.intercept('GET', '/api/users', { body: 'users.json' })

C. cy.intercept('GET', '/api/users', { fixture: 'users.json' })

D. cy.route('GET', '/api/users', 'fixture:users.json')

Solution

1. Step 1: Recall Cypress intercept syntax for fixture mocking

   The correct way is to use cy.intercept with method, URL, and an object with fixture property.

2. Step 2: Match the correct option

   cy.intercept('GET', '/api/users', { fixture: 'users.json' }) uses cy.intercept with { fixture: 'users.json' }, which is the correct syntax.

3. Final Answer:

   cy.intercept('GET', '/api/users', { fixture: 'users.json' }) -> Option C

4. Quick Check:

   Use cy.intercept + fixture option [OK]

Hint: Use cy.intercept with { fixture: 'file.json' } [OK]

Common Mistakes:

• Using cy.route instead of cy.intercept (deprecated)
• Passing fixture file as body string
• Using cy.mock which doesn't exist

3. Given the following Cypress code snippet, what will be the response body when the app requests '/api/products'?

cy.intercept('GET', '/api/products', { fixture: 'products.json' })
cy.visit('/products')

Assume 'products.json' contains [{"id":1,"name":"Book"}].

medium

A. [{"id":1,"name":"Book"}]

B. An empty array []

C. A network error occurs

D. The real server response for /api/products

Solution

1. Step 1: Understand intercept with fixture

   The intercept replaces the GET /api/products response with the content of 'products.json'.

2. Step 2: Check fixture content

   The fixture file contains [{"id":1,"name":"Book"}], so this will be the mocked response body.

3. Final Answer:

   [{"id":1,"name":"Book"}] -> Option A

4. Quick Check:

   Fixture content = mocked response body [OK]

Hint: Fixture content becomes the mocked response body [OK]

Common Mistakes:

• Expecting real server data instead of fixture
• Assuming empty array if fixture not found
• Thinking intercept causes network error

4. Identify the error in this Cypress code snippet that tries to mock a POST request with fixture 'newUser.json':

cy.intercept('POST', '/api/users', { fixture: 'newUser.json' })
cy.visit('/users')

medium

A. POST requests cannot be mocked with fixtures

B. The fixture file name must not include the .json extension

C. cy.visit should come before cy.intercept

D. The fixture option should be inside a response object, not directly in intercept

Solution

1. Step 1: Review intercept syntax for POST with fixture

   For POST requests, the fixture must be inside a response object like { body: { fixture: 'file' } } or use { fixture: 'file' } directly as response.

2. Step 2: Identify the mistake

   Passing { fixture: 'newUser.json' } directly as the third argument is correct syntax in Cypress 12+, so no error there. But if the Cypress version is older, it requires response object. However, the main common error is misunderstanding the fixture placement.

3. Step 3: Check other options

   POST requests can be mocked with fixtures, file extension is allowed, and cy.intercept should come before cy.visit.

4. Final Answer:

   The fixture option should be inside a response object, not directly in intercept -> Option D

5. Quick Check:

   Fixture must be correctly placed in intercept options [OK]

Hint: Place fixture inside response object for POST mocks [OK]

Common Mistakes:

• Thinking POST cannot be mocked with fixtures
• Ignoring correct order of cy.intercept and cy.visit
• Removing .json extension from fixture filename

5. You want to mock multiple API endpoints in a single Cypress test using fixtures: '/api/users' with 'users.json' and '/api/orders' with 'orders.json'. Which is the best way to do this?

hard

A. Mock only one endpoint and let the other call the real server

B. Call cy.intercept twice, once for each endpoint with its fixture

C. Use cy.intercept with an array of URLs and a single fixture

D. Combine both fixtures into one file and mock a single endpoint

Solution

1. Step 1: Understand mocking multiple endpoints

   Each endpoint requires its own intercept call to specify the URL and fixture separately.

2. Step 2: Evaluate options

   Call cy.intercept twice, once for each endpoint with its fixture correctly calls cy.intercept twice, once per endpoint, which is the best practice.

3. Step 3: Reject other options

   Combining fixtures or URLs is not supported; mocking only one endpoint leaves tests unreliable.

4. Final Answer:

   Call cy.intercept twice, once for each endpoint with its fixture -> Option B

5. Quick Check:

   One intercept per endpoint with fixture [OK]

Hint: Use separate cy.intercept calls per endpoint [OK]

Common Mistakes:

• Trying to mock multiple URLs in one intercept call
• Combining fixtures into one file unnecessarily
• Mocking only some endpoints and ignoring others