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Mock Functions

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.

info

The TypeScript examples from this page will only work as documented if you explicitly import Jest APIs:

import {expect, jest, test} from '@jest/globals';

Methods


Reference

mockFn.getMockName()

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

mockFn.mock.calls

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:

[
['arg1', 'arg2'],
['arg3', 'arg4'],
];

mockFn.mock.results

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:

  • 'return' - Indicates that the call completed by returning normally.
  • 'throw' - Indicates that the call completed by throwing a value.
  • 'incomplete' - Indicates that the call has not yet completed. This occurs if you test the result from within the mock function itself, or from within a function that was called by the mock.

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:

[
{
type: 'return',
value: 'result1',
},
{
type: 'throw',
value: {
/* Error instance */
},
},
{
type: 'return',
value: 'result2',
},
];

mockFn.mock.instances

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:

const mockFn = jest.fn();

const a = new mockFn();
const b = new mockFn();

mockFn.mock.instances[0] === a; // true
mockFn.mock.instances[1] === b; // true

mockFn.mock.contexts

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:

const mockFn = jest.fn();

const boundMockFn = mockFn.bind(thisContext0);
boundMockFn('a', 'b');
mockFn.call(thisContext1, 'a', 'b');
mockFn.apply(thisContext2, ['a', 'b']);

mockFn.mock.contexts[0] === thisContext0; // true
mockFn.mock.contexts[1] === thisContext1; // true
mockFn.mock.contexts[2] === thisContext2; // true

mockFn.mock.lastCall

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:

['arg3', 'arg4'];

mockFn.mockClear()

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.

Beware that mockFn.mockClear() will replace mockFn.mock, not just reset the values of its properties! You should, therefore, avoid assigning mockFn.mock to other variables, temporary or not, to make sure you don't access stale data.

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

mockFn.mockReset()

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. (Note that resetting a spy will result in a function with no return value).

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

mockFn.mockRestore()

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.

Beware that mockFn.mockRestore() only works when the mock was created with jest.spyOn(). Thus you have to take care of restoration yourself when manually assigning jest.fn().

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

mockFn.mockImplementation(fn)

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.

tip

jest.fn(implementation) is a shorthand for jest.fn().mockImplementation(implementation).

const mockFn = jest.fn(scalar => 42 + scalar);

mockFn(0); // 42
mockFn(1); // 43

mockFn.mockImplementation(scalar => 36 + scalar);

mockFn(2); // 38
mockFn(3); // 39

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

SomeClass.js
module.exports = class SomeClass {
method(a, b) {}
};
SomeClass.test.js
const SomeClass = require('./SomeClass');

jest.mock('./SomeClass'); // this happens automatically with automocking

const mockMethod = jest.fn();
SomeClass.mockImplementation(() => {
return {
method: mockMethod,
};
});

const some = new SomeClass();
some.method('a', 'b');

console.log('Calls to method: ', mockMethod.mock.calls);

mockFn.mockImplementationOnce(fn)

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.

const mockFn = jest
.fn()
.mockImplementationOnce(cb => cb(null, true))
.mockImplementationOnce(cb => cb(null, false));

mockFn((err, val) => console.log(val)); // true
mockFn((err, val) => console.log(val)); // false

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:

const mockFn = jest
.fn(() => 'default')
.mockImplementationOnce(() => 'first call')
.mockImplementationOnce(() => 'second call');

mockFn(); // 'first call'
mockFn(); // 'second call'
mockFn(); // 'default'
mockFn(); // 'default'

mockFn.mockName(name)

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

For example:

const mockFn = jest.fn().mockName('mockedFunction');

// mockFn();
expect(mockFn).toHaveBeenCalled();

Will result in this error:

expect(mockedFunction).toHaveBeenCalled()

Expected number of calls: >= 1
Received number of calls: 0

mockFn.mockReturnThis()

Shorthand for:

jest.fn(function () {
return this;
});

mockFn.mockReturnValue(value)

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

const mock = jest.fn();

mock.mockReturnValue(42);
mock(); // 42

mock.mockReturnValue(43);
mock(); // 43

mockFn.mockReturnValueOnce(value)

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.

const mockFn = jest
.fn()
.mockReturnValue('default')
.mockReturnValueOnce('first call')
.mockReturnValueOnce('second call');

mockFn(); // 'first call'
mockFn(); // 'second call'
mockFn(); // 'default'
mockFn(); // 'default'

mockFn.mockResolvedValue(value)

Shorthand for:

jest.fn().mockImplementation(() => Promise.resolve(value));

Useful to mock async functions in async tests:

test('async test', async () => {
const asyncMock = jest.fn().mockResolvedValue(43);

await asyncMock(); // 43
});

mockFn.mockResolvedValueOnce(value)

Shorthand for:

jest.fn().mockImplementationOnce(() => Promise.resolve(value));

Useful to resolve different values over multiple async calls:

test('async test', async () => {
const asyncMock = jest
.fn()
.mockResolvedValue('default')
.mockResolvedValueOnce('first call')
.mockResolvedValueOnce('second call');

await asyncMock(); // 'first call'
await asyncMock(); // 'second call'
await asyncMock(); // 'default'
await asyncMock(); // 'default'
});

mockFn.mockRejectedValue(value)

Shorthand for:

jest.fn().mockImplementation(() => Promise.reject(value));

Useful to create async mock functions that will always reject:

test('async test', async () => {
const asyncMock = jest
.fn()
.mockRejectedValue(new Error('Async error message'));

await asyncMock(); // throws 'Async error message'
});

mockFn.mockRejectedValueOnce(value)

Shorthand for:

jest.fn().mockImplementationOnce(() => Promise.reject(value));

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

test('async test', async () => {
const asyncMock = jest
.fn()
.mockResolvedValueOnce('first call')
.mockRejectedValueOnce(new Error('Async error message'));

await asyncMock(); // 'first call'
await asyncMock(); // throws 'Async error message'
});

mockFn.withImplementation(fn, callback)

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

test('test', () => {
const mock = jest.fn(() => 'outside callback');

mock.withImplementation(
() => 'inside callback',
() => {
mock(); // 'inside callback'
},
);

mock(); // 'outside callback'
});

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.

test('async test', async () => {
const mock = jest.fn(() => 'outside callback');

// We await this call since the callback is async
await mock.withImplementation(
() => 'inside callback',
async () => {
mock(); // 'inside callback'
},
);

mock(); // 'outside callback'
});

TypeScript Usage

tip

Please consult the Getting Started guide for details on how to setup Jest with TypeScript.

jest.fn(implementation?)

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):

import {expect, jest, test} from '@jest/globals';
import type add from './add';
import calculate from './calc';

test('calculate calls add', () => {
// Create a new mock that can be used in place of `add`.
const mockAdd = jest.fn<typeof add>();

// `.mockImplementation()` now can infer that `a` and `b` are `number`
// and that the returned value is a `number`.
mockAdd.mockImplementation((a, b) => {
// Yes, this mock is still adding two numbers but imagine this
// was a complex function we are mocking.
return a + b;
});

// `mockAdd` is properly typed and therefore accepted by anything
// requiring `add`.
calculate(mockAdd, 1, 2);

expect(mockAdd).toBeCalledTimes(1);
expect(mockAdd).toBeCalledWith(1, 2);
});

jest.Mock<T>

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:

import {jest} from '@jest/globals';

const sumRecursively: jest.Mock<(value: number) => number> = jest.fn(value => {
if (value === 0) {
return 0;
} else {
return value + fn(value - 1);
}
});

jest.Mocked<Source>

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

import {expect, jest, test} from '@jest/globals';
import type {fetch} from 'node-fetch';

jest.mock('node-fetch');

let mockedFetch: jest.Mocked<typeof fetch>;

afterEach(() => {
mockedFetch.mockClear();
});

test('makes correct call', () => {
mockedFetch = getMockedFetch();
// ...
});

test('returns correct data', () => {
mockedFetch = getMockedFetch();
// ...
});

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

jest.mocked(source, options?)

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.

song.ts
export const song = {
one: {
more: {
time: (t: number) => {
return t;
},
},
},
};
song.test.ts
import {expect, jest, test} from '@jest/globals';
import {song} from './song';

jest.mock('./song');
jest.spyOn(console, 'log');

const mockedSong = jest.mocked(song);
// or through `jest.Mocked<Source>`
// const mockedSong = song as jest.Mocked<typeof song>;

test('deep method is typed correctly', () => {
mockedSong.one.more.time.mockReturnValue(12);

expect(mockedSong.one.more.time(10)).toBe(12);
expect(mockedSong.one.more.time.mock.calls).toHaveLength(1);
});

test('direct usage', () => {
jest.mocked(console.log).mockImplementation(() => {
return;
});

console.log('one more time');

expect(jest.mocked(console.log).mock.calls).toHaveLength(1);
});