Mocking and Test Doubles

How each unit testing framework supports mocks, stubs, and spies.

Jest

Ships comprehensive built-in mocking (jest.fn(), jest.mock(), module mocking, timer mocking) with no separate library needed.

Vitest

Ships Jest-compatible built-in mocking (vi.fn(), vi.mock()), including module and timer mocking, mirroring Jest’s API closely.

Mocha

Has no built-in mocking at all — teams typically add Sinon.js for spies, stubs, and mocks alongside their chosen assertion library.

pytest

Relies on Python’s built-in unittest.mock module (commonly used via the pytest-mock plugin for cleaner fixture-based integration) rather than shipping its own mocking system.

JUnit

Has no built-in mocking — Mockito is the near-universal standard companion library for mocks and stubs in the Java ecosystem.

RSpec

Ships built-in “test doubles” and message expectations (allow/expect(...).to receive) as a first-class part of the framework itself.

Go testing

Has no built-in mocking; Go’s interface-based design makes hand-written fakes common, with libraries like Testify’s mock package or generated mocks (via mockgen) as popular alternatives.

xUnit

Has no built-in mocking; Moq or NSubstitute are the most common companion mocking libraries in the .NET ecosystem.

PHPUnit

Ships a built-in mocking API (createMock(), createStub()) as part of the core framework, with no separate library required for typical use.

Rust built-in tests

Has no built-in mocking framework; Rust’s trait system makes hand-written fakes straightforward, with crates like mockall available for more automated mock generation.