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**Note:** More details on user side API can be found in [Jest documentation](


Version published
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Weekly downloads





  • [jest-changed-files] Support Sapling (#13941)
  • [jest-circus, @jest/cli, jest-config] Add feature to randomize order of tests via CLI flag or through the config file(#12922)
  • [jest-cli, jest-config, @jest/core, jest-haste-map, @jest/reporters, jest-runner, jest-runtime, @jest/types] Add workerThreads configuration option to allow using worker threads for parallelization (#13939)
  • [jest-cli] Export yargsOptions (#13970)
  • [jest-config] Add openHandlesTimeout option to configure possible open handles warning. (#13875)
  • [@jest/create-cache-key-function] Allow passing length argument to createCacheKey() function and set its default value to 16 on Windows (#13827)
  • [jest-message-util] Add support for AggregateError (#13946 & #13947)
  • [jest-message-util] Add support for Error causes in test and it (#13935 & #13966)
  • [jest-reporters] Add summaryThreshold option to summary reporter to allow overriding the internal threshold that is used to print the summary of all failed tests when the number of test suites surpasses it (#13895)
  • [jest-runtime] Expose @sinonjs/fake-timers async APIs functions advanceTimersByTimeAsync(msToRun) (tickAsync(msToRun)), advanceTimersToNextTimerAsync(steps) (nextAsync), runAllTimersAsync (runAllAsync), and runOnlyPendingTimersAsync (runToLastAsync) (#13981)
  • [jest-runtime, @jest/transform] Allow V8 coverage provider to collect coverage from files which were not loaded explicitly (#13974)
  • [jest-snapshot] Add support to cts and mts TypeScript files to inline snapshots (#13975)
  • [jest-worker] Add start method to worker farms (#13937)
  • [jest-worker] Support passing a URL as path to worker (#13982)


  • [babel-plugin-jest-hoist] Fix unwanted hoisting of nested jest usages (#13952)
  • [jest-circus] Send test case results for todo tests (#13915)
  • [jest-circus] Update message printed on test timeout (#13830)
  • [jest-circus] Avoid creating the word "testfalse" when takesDoneCallback is false in the message printed on test timeout AND updated timeouts test (#13954)
  • [jest-environment-jsdom] Stop setting document to null on teardown (#13972)
  • [@jest/expect-utils] Update toStrictEqual() to be able to check jest.fn().mock.calls (#13960)
  • [@jest/test-result] Allow TestResultsProcessor type to return a Promise (#13950)

Chore & Maintenance

  • [jest-snapshot] Remove dependency on jest-haste-map (#13977)




Note: More details on user side API can be found in Jest documentation.


import {ModuleMocker} from 'jest-mock';


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


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.


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.


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:


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.


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


Sets the default return value for the function.


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


Sets the default mock implementation for the function.


Syntactic sugar for:

mockFn.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.
.withImplementation(function, callback)

Temporarily overrides the default mock implementation within the callback, then restores it's previous implementation.

If the callback is async or returns a thenable, withImplementation will return a promise. Awaiting the promise will await the callback and reset the implementation.


Last updated on 06 Mar 2023

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