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Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than only testing the output. You can create a mock function with jest.fn(). If no implementation is given, the mock function will return undefined when invoked.

Returns the mock name string set by calling .mockName().

An array containing the call arguments of all calls that have been made to this mock function. Each item in the array is an array of arguments that were passed during the call.

For example: A mock function f that has been called twice, with the arguments f('arg1', 'arg2'), and then with the arguments f('arg3', 'arg4'), would have a mock.calls array that looks like this:

An array containing the results of all calls that have been made to this mock function. Each entry in this array is an object containing a type property, and a value property. type will be one of the following:

The value property contains the value that was thrown or returned. value is undefined when type === 'incomplete'.

For example: A mock function f that has been called three times, returning 'result1', throwing an error, and then returning 'result2', would have a mock.results array that looks like this:

An array that contains all the object instances that have been instantiated from this mock function using new.

For example: A mock function that has been instantiated twice would have the following mock.instances array:

An array that contains the contexts for all calls of the mock function.

A context is the this value that a function receives when called. The context can be set using Function.prototype.bind, Function.prototype.call or Function.prototype.apply.

For example:

An array containing the call arguments of the last call that was made to this mock function. If the function was not called, it will return undefined.

For example: A mock function f that has been called twice, with the arguments f('arg1', 'arg2'), and then with the arguments f('arg3', 'arg4'), would have a mock.lastCall array that looks like this:

Clears all information stored in the mockFn.mock.calls, mockFn.mock.instances, mockFn.mock.contexts and mockFn.mock.results arrays. Often this is useful when you want to clean up a mocks usage data between two assertions.

The clearMocks configuration option is available to clear mocks automatically before each tests.

Does everything that mockFn.mockClear() does, and also removes any mocked return values or implementations.

This is useful when you want to completely reset a mock back to its initial state.

The resetMocks configuration option is available to reset mocks automatically before each test.

Does everything that mockFn.mockReset() does, and also restores the original (non-mocked) implementation.

This is useful when you want to mock functions in certain test cases and restore the original implementation in others.

The restoreMocks configuration option is available to restore mocks automatically before each test.

Accepts a function that should be used as the implementation of the mock. The mock itself will still record all calls that go into and instances that come from itself – the only difference is that the implementation will also be executed when the mock is called.

.mockImplementation() can also be used to mock class constructors:

Accepts a function that will be used as an implementation of the mock for one call to the mocked function. Can be chained so that multiple function calls produce different results.

When the mocked function runs out of implementations defined with .mockImplementationOnce(), it will execute the default implementation set with jest.fn(() => defaultValue) or .mockImplementation(() => defaultValue) if they were called:

Accepts a string to use in test result output in place of 'jest.fn()' to indicate which mock function is being referenced.

For example:

Will result in this error:

Shorthand for:

Shorthand for:

Accepts a value that will be returned whenever the mock function is called.

Shorthand for:

Accepts a value that will be returned for one call to the mock function. Can be chained so that successive calls to the mock function return different values. When there are no more mockReturnValueOnce values to use, calls will return a value specified by mockReturnValue.

Shorthand for:

Useful to mock async functions in async tests:

Shorthand for:

Useful to resolve different values over multiple async calls:

Shorthand for:

Useful to create async mock functions that will always reject:

Shorthand for:

Useful together with .mockResolvedValueOnce() or to reject with different exceptions over multiple async calls:

Accepts a function which should be temporarily used as the implementation of the mock while the callback is being executed.

mockFn.withImplementation can be used regardless of whether or not the callback is asynchronous (returns a thenable). If the callback is asynchronous a promise will be returned. Awaiting the promise will await the callback and reset the implementation.

Changes the value of already replaced property. This is useful when you want to replace property and then adjust the value in specific tests. As an alternative, you can call jest.replaceProperty() multiple times on same property.

Restores object's property to the original value.

Beware that replacedProperty.restore() only works when the property value was replaced with jest.replaceProperty().

The restoreMocks configuration option is available to restore replaced properties automatically before each test.

Correct mock typings will be inferred if implementation is passed to jest.fn(). There are many use cases where the implementation is omitted. To ensure type safety you may pass a generic type argument (also see the examples above for more reference):

Constructs the type of a mock function, e.g. the return type of jest.fn(). It can be useful if you have to defined a recursive mock function:

The jest.Mocked utility type returns the Source type wrapped with type definitions of Jest mock function.

Types of classes, functions or objects can be passed as type argument to jest.Mocked. If you prefer to constrain the input type, use: jest.MockedClass, jest.MockedFunction or jest.MockedObject.

The jest.Replaced utility type returns the Source type wrapped with type definitions of Jest replaced property.

The mocked() helper method wraps types of the source object and its deep nested members with type definitions of Jest mock function. You can pass {shallow: true} as the options argument to disable the deeply mocked behavior.

Returns the source object.

Constructs the type of a spied class or function (i.e. the return type of jest.spyOn()).