JavaScript Testing 3 — Questions and Answers
Question 1: What does TDD stand for and what is its core cycle?
- Test-Driven Development; Red-Green-Refactor (Correct answer)
- Type-Driven Design; Plan-Code-Test
- Test-Debug-Deploy; Write-Run-Fix
- Test-Driven Design; Mock-Assert-Verify
Correct answer: Test-Driven Development; Red-Green-Refactor
TDD stands for Test-Driven Development and follows the Red (failing test) → Green (make it pass) → Refactor cycle.
Question 2: When using `jest.spyOn(object, 'method')`, what happens to the original method by default?
- It is replaced with a no-op function
- It is called through (original behavior preserved) (Correct answer)
- It throws an error if called
- It returns undefined always
Correct answer: It is called through (original behavior preserved)
jest.spyOn wraps the original method so it can be observed, but the original implementation still executes by default.
Question 3: Which Cypress command waits for an element to be visible before interacting with it?
- cy.find()
- cy.get() (automatically) (Correct answer)
- cy.wait()
- cy.exists()
Correct answer: cy.get() (automatically)
cy.get() automatically retries and waits for elements to exist and be interactable before the command proceeds.
Question 4: What is snapshot testing primarily used for in Jest?
- Performance benchmarking
- Detecting unintended UI or output changes (Correct answer)
- Testing database snapshots
- Capturing network responses
Correct answer: Detecting unintended UI or output changes
Snapshot testing serializes a component's output and compares it against a stored snapshot to catch unexpected changes.
Question 5: What is the difference between a stub and a spy in testing?
- Stubs record calls; spies replace implementations
- Spies record calls; stubs replace implementations with preset responses (Correct answer)
- They are identical concepts
- Stubs are for async code; spies are for sync code
Correct answer: Spies record calls; stubs replace implementations with preset responses
A spy records how a function is called while preserving behavior; a stub replaces a function with a controlled return value.
Question 6: How do you test that a Jest mock was called with specific arguments?
- mockFn.assert(args)
- expect(mockFn).toHaveBeenCalledWith(args) (Correct answer)
- mockFn.verify(args)
- expect(mockFn.args).toEqual(args)
Correct answer: expect(mockFn).toHaveBeenCalledWith(args)
expect(mockFn).toHaveBeenCalledWith(args) asserts the mock was called at least once with the specified arguments.
Question 7: What does `expect.assertions(n)` do in a Jest test?
- Limits the test to n assertions
- Verifies that exactly n assertions were called during the test (Correct answer)
- Skips the test if fewer than n assertions exist
- Sets test timeout based on n
Correct answer: Verifies that exactly n assertions were called during the test
expect.assertions(n) ensures the test fails if the expected number of assertions is not reached, useful for async tests.
JavaScript Testing 3
What does TDD stand for and what is its core cycle?