jest-mock
Advanced tools
Package description
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.
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');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 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.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.
Changelog
24.9.0
[expect] Highlight substring differences when matcher fails, part 1 (#8448)[expect] Highlight substring differences when matcher fails, part 2 (#8528)[expect] Improve report when mock-spy matcher fails, part 1 (#8640)[expect] Improve report when mock-spy matcher fails, part 2 (#8649)[expect] Improve report when mock-spy matcher fails, part 3 (#8697)[expect] Improve report when mock-spy matcher fails, part 4 (#8710)[expect] Throw matcher error when received cannot be jasmine spy (#8747)[expect] Improve report when negative CalledWith assertion fails (#8755)[expect] Improve report when positive CalledWith assertion fails (#8771)[expect] Display equal values for ReturnedWith similar to CalledWith (#8791)[expect, jest-snapshot] Change color from green for some args in matcher hints (#8812)[jest-snapshot] Highlight substring differences when matcher fails, part 3 (#8569)[jest-core] Improve report when snapshots are obsolete (#8665)[jest-cli] Improve chai support (with detailed output, to match jest exceptions) (#8454)[*] Manage the global timeout with --testTimeout command line argument. (#8456)[pretty-format] Render custom displayName of memoized components (#8546)[jest-validate] Allow maxWorkers as part of the jest.config.js (#8565)[jest-runtime] Allow passing configuration objects to transformers (#7288)[@jest/core, @jest/test-sequencer] Support async sort in custom testSequencer (#8642)[jest-runtime, @jest/fake-timers] Add jest.advanceTimersToNextTimer (#8713)[@jest-transform] Extract transforming require logic within jest-core into @jest-transform (#8756)[jest-matcher-utils] Add color options to matcherHint (#8795)[jest-circus/jest-jasmine2] Give clearer output for Node assert errors (#8792)[jest-runner] Export all types in the type signature of jest-runner (#8825)[jest-cli] Detect side-effect only imports when running --onlyChanged or --changedSince (#8670)[jest-cli] Allow --maxWorkers to work with % input again (#8565)[babel-plugin-jest-hoist] Expand list of whitelisted globals in global mocks (#8429)[jest-core] Make watch plugin initialization errors look nice (#8422)[jest-snapshot] Prevent inline snapshots from drifting when inline snapshots are updated (#8492)[jest-haste-map] Don't throw on missing mapper in Node crawler (#8558)[jest-core] Fix incorrect passWithNoTests warning (#8595)[jest-snapshots] Fix test retries that contain snapshots (#8629)[jest-mock] Fix incorrect assignments when restoring mocks in instances where they originally didn't exist (#8631)[expect] Fix stack overflow when matching objects with circular references (#8687)[jest-haste-map] Workaround a node >=12.5.0 bug that causes the process not to exit after tests have completed and cancerous memory growth (#8787)[docs] Replace FlowType with TypeScript in CONTRIBUTING.MD code conventions[jest-leak-detector] remove code repeat (#8438)[docs] Add example to jest.requireActual (#8482)[docs] Add example to jest.mock for mocking ES6 modules with the factory parameter (#8550)[docs] Add information about using jest.doMock with ES6 imports (#8573)[docs] Fix variable name in custom-matcher-api code example (#8582)[docs] Fix example used in custom environment docs (#8617)[docs] Updated react tutorial to refer to new package of react-testing-library (@testing-library/react) (#8753)[docs] Updated imports of react-testing-library to @testing-library/react in website (#8757)[jest-core] Add getVersion (moved from jest-cli) (#8706)[docs] Fix MockFunctions example that was using toContain instead of toContainEqual (#8765)[*] Make sure copyright header comment includes license (#8783)[*] Check copyright and license as one joined substring (#8815)[docs] Fix WatchPlugins jestHooks.shouldRunTestSuite example that receives an object (#8784)[*] Enforce LF line endings (#8809)[pretty-format] Delete obsolete link and simplify structure in README (#8824)[docs] Fix broken transform link on webpack page (#9155)Readme
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 simply by defining metadata for the member.prototype of the function. The type of a function prototype should always be object. For instance, a simple 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.
fnGenerates a stand-alone function with members that help drive unit tests or confirm expectations. Specifically, functions returned by this method have the following members:
.mockAn 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:
FAQs
**Note:** More details on user side API can be found in [Jest documentation](https://jestjs.io/docs/mock-function-api).
The npm package jest-mock receives a total of 23,214,838 weekly downloads. As such, jest-mock popularity was classified as popular.
We found that jest-mock demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 3 open source maintainers collaborating on the project.
Did you know?
Socket for GitHub automatically highlights issues in each pull request and monitors the health of all your open source dependencies. Discover the contents of your packages and block harmful activity before you install or update your dependencies.
Security News
ESLint has approved an RFC that adds support for TypeScript configuration files, which is aimed at improving the developer experience and recognizing changes in the evolving JavaScript ecosystem.
Security News
The NVD is facing a significant backlog with over 12,500 CVEs awaiting analysis, and more than 50% of known exploited vulnerabilities (KEVs) left unenriched since mid-February.
Security News
Ransomware costs victims an estimated $30 billion per year and has gotten so out of control that global support for banning payments is gaining momentum.