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Version: 29.1


Mock functions allow you to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with new, and allowing test-time configuration of return values.

There are two ways to mock functions: Either by creating a mock function to use in test code, or writing a manual mock to override a module dependency.

Использование mock-функции

Давайте представим, что мы тестируем реализацию функции forEach, которая выполняет обратный вызов для каждого элемента предоставленного массива.

export function forEach(items, callback) {
for (let index = 0; index < items.length; index++) {

Чтобы протестировать эту функцию, мы можем использовать мок-функцию, и посмотреть на состояние мока чтобы убедиться, что функция была вызвана как ожидалось.

const forEach = require('./forEach');

const mockCallback = jest.fn(x => 42 + x);

test('forEach mock function', () => {
forEach([0, 1], mockCallback);

// The mock function was called twice

// The first argument of the first call to the function was 0

// The first argument of the second call to the function was 1

// The return value of the first call to the function was 42

.mock свойство

У всех мок-функций есть особое свойство .mock, где хранятся данные о том как функция была вызвана и что она вернула. Свойство .mock также отслеживает значение this для каждого вызова, так что как правило это можно посмотреть:

const myMock1 = jest.fn();
const a = new myMock1();
// > [ <a> ]

const myMock2 = jest.fn();
const b = {};
const bound = myMock2.bind(b);
// > [ <b> ]

Эти свойства мока очень полезны в тестах чтобы указывать как эти функции вызываются, наследуются, или что они возвращают:

// The function was called exactly once

// The first arg of the first call to the function was 'first arg'
expect(someMockFunction.mock.calls[0][0]).toBe('first arg');

// The second arg of the first call to the function was 'second arg'
expect(someMockFunction.mock.calls[0][1]).toBe('second arg');

// The return value of the first call to the function was 'return value'
expect(someMockFunction.mock.results[0].value).toBe('return value');

// The function was called with a certain `this` context: the `element` object.

// This function was instantiated exactly twice

// The object returned by the first instantiation of this function
// had a `name` property whose value was set to 'test'

// The first argument of the last call to the function was 'test'

Значения возвращаемые имитаторами

Мок-функции также могут использоваться для внедрения тестовых значений в ваш код во время тестирования:

const myMock = jest.fn();
// > undefined


console.log(myMock(), myMock(), myMock(), myMock());
// > 10, 'x', true, true

Mock functions are also very effective in code that uses a functional continuation-passing style. Code written in this style helps avoid the need for complicated stubs that recreate the behavior of the real component they're standing in for, in favor of injecting values directly into the test right before they're used.

const filterTestFn = jest.fn();

// Make the mock return `true` for the first call,
// and `false` for the second call

const result = [11, 12].filter(num => filterTestFn(num));

// > [11]
console.log(filterTestFn.mock.calls[0][0]); // 11
console.log(filterTestFn.mock.calls[1][0]); // 12

Most real-world examples actually involve getting ahold of a mock function on a dependent component and configuring that, but the technique is the same. In these cases, try to avoid the temptation to implement logic inside of any function that's not directly being tested.

Мокинг (имитации) модулей

Предположим, у нас есть класс, который получает пользователей из нашего API. Этот класс использует axios для вызова API, а затем возвращает атрибут data, который содержит всех пользователей:

import axios from 'axios';

class Users {
static all() {
return axios.get('/users.json').then(resp => resp.data);

export default Users;

Now, in order to test this method without actually hitting the API (and thus creating slow and fragile tests), we can use the jest.mock(...) function to automatically mock the axios module.

Once we mock the module we can provide a mockResolvedValue for .get that returns the data we want our test to assert against. In effect, we are saying that we want axios.get('/users.json') to return a fake response.

import axios from 'axios';
import Users from './users';


test('should fetch users', () => {
const users = [{name: 'Bob'}];
const resp = {data: users};

// or you could use the following depending on your use case:
// axios.get.mockImplementation(() => Promise.resolve(resp))

return Users.all().then(data => expect(data).toEqual(users));

Mocking Partials

Subsets of a module can be mocked and the rest of the module can keep their actual implementation:

export const foo = 'foo';
export const bar = () => 'bar';
export default () => 'baz';
import defaultExport, {bar, foo} from '../foo-bar-baz';

jest.mock('../foo-bar-baz', () => {
const originalModule = jest.requireActual('../foo-bar-baz');

//Mock the default export and named export 'foo'
return {
__esModule: true,
default: jest.fn(() => 'mocked baz'),
foo: 'mocked foo',

test('should do a partial mock', () => {
const defaultExportResult = defaultExport();
expect(defaultExportResult).toBe('mocked baz');

expect(foo).toBe('mocked foo');

Реализации имитаторов

Still, there are cases where it's useful to go beyond the ability to specify return values and full-on replace the implementation of a mock function. This can be done with jest.fn or the mockImplementationOnce method on mock functions.

const myMockFn = jest.fn(cb => cb(null, true));

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

The mockImplementation method is useful when you need to define the default implementation of a mock function that is created from another module:

module.exports = function () {
// some implementation;
jest.mock('../foo'); // this happens automatically with automocking
const foo = require('../foo');

// foo is a mock function
foo.mockImplementation(() => 42);
// > 42

When you need to recreate a complex behavior of a mock function such that multiple function calls produce different results, use the mockImplementationOnce method:

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

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

myMockFn((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 (if it is defined):

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

console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn());
// > 'first call', 'second call', 'default', 'default'

For cases where we have methods that are typically chained (and thus always need to return this), we have a sugary API to simplify this in the form of a .mockReturnThis() function that also sits on all mocks:

const myObj = {
myMethod: jest.fn().mockReturnThis(),

// is the same as

const otherObj = {
myMethod: jest.fn(function () {
return this;

Mock Names

You can optionally provide a name for your mock functions, which will be displayed instead of 'jest.fn()' in the test error output. Use .mockName() if you want to be able to quickly identify the mock function reporting an error in your test output.

const myMockFn = jest
.mockImplementation(scalar => 42 + scalar)

Пользовательские матчеры

Finally, in order to make it less demanding to assert how mock functions have been called, we've added some custom matcher functions for you:

// The mock function was called at least once

// The mock function was called at least once with the specified args
expect(mockFunc).toHaveBeenCalledWith(arg1, arg2);

// The last call to the mock function was called with the specified args
expect(mockFunc).toHaveBeenLastCalledWith(arg1, arg2);

// All calls and the name of the mock is written as a snapshot

These matchers are sugar for common forms of inspecting the .mock property. You can always do this manually yourself if that's more to your taste or if you need to do something more specific:

// The mock function was called at least once

// The mock function was called at least once with the specified args
expect(mockFunc.mock.calls).toContainEqual([arg1, arg2]);

// The last call to the mock function was called with the specified args
expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1]).toEqual([

// The first arg of the last call to the mock function was `42`
// (note that there is no sugar helper for this specific of an assertion)
expect(mockFunc.mock.calls[mockFunc.mock.calls.length - 1][0]).toBe(42);

// A snapshot will check that a mock was invoked the same number of times,
// in the same order, with the same arguments. It will also assert on the name.
expect(mockFunc.mock.calls).toEqual([[arg1, arg2]]);
expect(mockFunc.getMockName()).toBe('a mock name');

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