jest-mock

What is jest-mock?

The jest-mock package is a module for mocking classes and objects in JavaScript tests. It is part of the Jest testing framework but can be used independently. It allows developers to create mock functions, spy on function calls, and set return values or implementations for testing purposes.

What are jest-mock's main functionalities?

Creating Mock Functions

This feature allows you to create a mock function that can track calls, arguments, and return values. It can be used to test how a function is used without actually executing its logic.

const mockFn = jest.fn();

Mocking Return Values

With this feature, you can set a default return value for a mock function. This is useful when you want to test the behavior of code that depends on the return value of the function being mocked.

const mockFn = jest.fn().mockReturnValue('default value');

Mocking Modules

This feature allows you to mock entire modules, which is particularly useful when you want to isolate the module being tested from its dependencies.

jest.mock('moduleName', () => { return jest.fn(() => 'mocked module') });

Spying on Methods

Spying on methods enables you to observe and track calls to object methods without affecting their actual implementation. This is useful for verifying that methods are called with the correct arguments.

const spy = jest.spyOn(object, 'methodName');

Other packages similar to jest-mock

sinon

Sinon is a standalone test spies, stubs, and mocks library for JavaScript. It provides similar functionalities to jest-mock, such as spying on functions, creating stubs, and mocking objects. Sinon can be used with any testing framework and is not tied to Jest.

proxyquire

Proxyquire is a tool for mocking modules in Node.js. It allows you to override dependencies during testing, which is similar to jest-mock's module mocking capabilities. However, proxyquire focuses specifically on Node.js module system and does not provide function mocking or spying.

testdouble

Testdouble.js (td.js) is a minimal test double library for TDD with JavaScript. It provides a similar API to jest-mock for creating test doubles like stubs, mocks, and spies. Testdouble.js aims to have a simpler and more expressive syntax compared to other mocking libraries.

README

jest-mock

API

constructor(global)

Creates a new module mocker that generates mocks as if they were created in an environment with the given global object.

generateFromMetadata(metadata)

Generates a mock based on the given metadata (Metadata for the mock in the schema returned by the getMetadata method of this module). Mocks treat functions specially, and all mock functions have additional members, described in the documentation for fn in this module.

One important note: function prototypes are handled specially by this mocking framework. For functions with prototypes, when called as a constructor, the mock will install mocked function members on the instance. This allows different instances of the same constructor to have different values for its mocks member and its return values.

getMetadata(component)

Inspects the argument and returns its schema in the following recursive format:

{
  type: ...
  members: {}
}

Where type is one of array, object, function, or ref, and members is an optional dictionary where the keys are member names and the values are metadata objects. Function prototypes are defined by defining metadata for the member.prototype of the function. The type of a function prototype should always be object. For instance, a class might be defined like this:

const classDef = {
  type: 'function',
  members: {
    staticMethod: {type: 'function'},
    prototype: {
      type: 'object',
      members: {
        instanceMethod: {type: 'function'},
      },
    },
  },
};

Metadata may also contain references to other objects defined within the same metadata object. The metadata for the referent must be marked with refID key and an arbitrary value. The referrer must be marked with a ref key that has the same value as object with refID that it refers to. For instance, this metadata blob:

const refID = {
  type: 'object',
  refID: 1,
  members: {
    self: {ref: 1},
  },
};

defines an object with a slot named self that refers back to the object.

fn

Generates a stand-alone function with members that help drive unit tests or confirm expectations. Specifically, functions returned by this method have the following members:

.mock

An object with three members, calls, instances and invocationCallOrder, which are all lists. The items in the calls list are the arguments with which the function was called. The "instances" list stores the value of 'this' for each call to the function. This is useful for retrieving instances from a constructor. The invocationCallOrder lists the order in which the mock was called in relation to all mock calls, starting at 1.

.mockReturnValueOnce(value)

Pushes the given value onto a FIFO queue of return values for the function.

.mockReturnValue(value)

Sets the default return value for the function.

.mockImplementationOnce(function)

Pushes the given mock implementation onto a FIFO queue of mock implementations for the function.

.mockImplementation(function)

Sets the default mock implementation for the function.

.mockReturnThis()

Syntactic sugar for .mockImplementation(function() {return this;})

In case both mockImplementationOnce()/mockImplementation() and mockReturnValueOnce()/mockReturnValue() are called. The priority of which to use is based on what is the last call:

• if the last call is mockReturnValueOnce() or mockReturnValue(), use the specific return value or default return value. If specific return values are used up or no default return value is set, fall back to try mockImplementation();
• if the last call is mockImplementationOnce() or mockImplementation(), run the specific implementation and return the result or run default implementation and return the result.

Changelog

25.5.0

Features

• [@jest/globals] New package so Jest's globals can be explicitly imported (#9801)
• [jest-core] Show coverage of sources related to tests in changed files (#9769)
• [jest-runtime] Populate require.cache (#9841)

Fixes

• [*] Use graceful-fs directly in every package instead of relying on fs being monkey patched (#9443)
• [expect] Prints the Symbol name into the error message with a custom asymmetric matcher (#9888)
• [jest-circus, jest-jasmine2] Support older version of jest-runtime (#9903 & #9842)
• [@jest/environment] Make sure not to reference Jest types (#9875)
• [jest-message-util] Code frame printing should respect --noStackTrace flag (#9866)
• [jest-runtime] Support importing CJS from ESM using import statements (#9850)
• [jest-runtime] Support importing parallel dynamic imports (#9858)
• [jest-transform] Improve source map handling when instrumenting transformed code (#9811)

Chore & Maintenance

• [docs] Add an example for mocking non-default export class

Performance

• [jest-resolve] Update resolve to a version using native realpath, which is faster than the default JS implementation (#9872)
• [jest-resolve] Pass custom cached realpath function to resolve (#9873)
• [jest-runtime] Add teardown method to clear any caches when tests complete (#9906)
• [jest-runtime] Do not pass files required internally through transformation when loading them (#9900)
• [jest-runtime] Use Maps instead of object literals as cache holders (#9901)