TypeScript icon, indicating that this package has built-in type declarations

3.0.7 • Public • Published


Type safe mocking extensions for Jest 🃏

Build Status Coverage Status npm version License: MIT npm downloads


  • Provides complete Typescript type safety for interfaces, argument types and return types
  • Ability to mock any interface or object
  • calledWith() extension to provide argument specific expectations, which works for objects and functions.
  • Extensive Matcher API compatible with Jasmine matchers
  • Supports mocking deep objects / class instances.
  • Familiar Jest like API


npm install jest-mock-extended --save-dev


yarn add jest-mock-extended --dev


import { mock } from 'jest-mock-extended';

interface PartyProvider {
   getPartyType: () => string;
   getSongs: (type: string) => string[]
   start: (type: string) => void;

describe('Party Tests', () => {
   test('Mock out an interface', () => {
       const mock = mock<PartyProvider>();
       mock.start('disco party');
       expect(mock.start).toHaveBeenCalledWith('disco party');
   test('mock out a return type', () => {
       const mock = mock<PartyProvider>();
       mock.getPartyType.mockReturnValue('west coast party');
       expect(mock.getPartyType()).toBe('west coast party');

    test('throwing an error if we forget to specify the return value')
        const mock = mock<PartyProvider>(
                fallbackMockImplementation: () => {
                    throw new Error('not mocked');

        expect(() => mock.getPartyType()).toThrowError('not mocked');

Assigning Mocks with a Type

If you wish to assign a mock to a variable that requires a type in your test, then you should use the MockProxy<> type given that this will provide the apis for calledWith() and other built-in jest types for providing test functionality.

import { MockProxy, mock } from 'jest-mock-extended';

describe('test', () => {
    let myMock: MockProxy<MyInterface>;

    beforeEach(() => {
        myMock = mock<MyInterface>();

    test(() => {

calledWith() Extension

jest-mock-extended allows for invocation matching expectations. Types of arguments, even when using matchers are type checked.

const provider = mock<PartyProvider>();
provider.getSongs.calledWith('disco party').mockReturnValue(['Dance the night away', 'Stayin Alive']);
expect(provider.getSongs('disco party')).toEqual(['Dance the night away', 'Stayin Alive']);

// Matchers
provider.getSongs.calledWith(any()).mockReturnValue(['Saw her standing there']);
provider.getSongs.calledWith(anyString()).mockReturnValue(['Saw her standing there']);

You can also use mockFn() to create a jest.fn() with the calledWith extension:

 type MyFn = (x: number, y: number) => Promise<string>;
 const fn = mockFn<MyFn>();
 fn.calledWith(1, 2).mockReturnValue('str');

Clearing / Resetting Mocks

jest-mock-extended exposes a mockClear and mockReset for resetting or clearing mocks with the same functionality as jest.fn().

import { mock, mockClear, mockReset } from 'jest-mock-extended';

describe('test', () => {
   const mock: UserService = mock<UserService>();
   beforeEach(() => {
      mockReset(mock); // or mockClear(mock)

Deep mocks

If your class has objects returns from methods that you would also like to mock, you can use mockDeep in replacement for mock.

import { mockDeep } from 'jest-mock-extended';

const mockObj: DeepMockProxy<Test1> = mockDeep<Test1>();

if you also need support for properties on functions, you can pass in an option to enable this

import { mockDeep } from 'jest-mock-extended';

const mockObj: DeepMockProxy<Test1> = mockDeep<Test1>({ funcPropSupport: true });


Can can provide a fallback mock implementation used if you do not define a return value using calledWith.

import { mockDeep } from 'jest-mock-extended';
const mockObj = mockDeep<Test1>({
    fallbackMockImplementation: () => {
        throw new Error('please add expected return value using calledWith');
expect(() => mockObj.getNumber()).toThrowError('not mocked');

Available Matchers

Matcher Description
any() Matches any arg of any type.
anyBoolean() Matches any boolean (true or false)
anyString() Matches any string including empty string
anyNumber() Matches any number that is not NaN
anyFunction() Matches any function
anyObject() Matches any object (typeof m === 'object') and is not null
anyArray() Matches any array
anyMap() Matches any Map
anySet() Matches any Set
isA(class) e.g isA(DiscoPartyProvider)
includes('value') Checks if value is in the argument array
containsKey('key') Checks if the key exists in the object
containsValue('value') Checks if the value exists in an object
has('value') checks if the value exists in a Set
notNull() value !== null
notUndefined() value !== undefined
notEmpty() value !== undefined && value !== null && value !== ''
captor() Used to capture an arg - alternative to mock.calls[0][0]

Writing a Custom Matcher

Custom matchers can be written using a MatcherCreator

import { MatcherCreator, Matcher } from 'jest-mock-extended';

// expectedValue is optional
export const myMatcher: MatcherCreator<MyType> = (expectedValue) => new Matcher((actualValue) => {
    return (expectedValue === actualValue && actualValue.isSpecial);

By default, the expected value and actual value are the same type. In the case where you need to type the expected value differently than the actual value, you can use the optional 2 generic parameter:

import { MatcherCreator, Matcher } from 'jest-mock-extended';

// expectedValue is optional
export const myMatcher: MatcherCreator<string[], string> = (expectedValue) => new Matcher((actualValue) => {
    return (actualValue.includes(expectedValue));




Package Sidebar


npm i jest-mock-extended

Weekly Downloads






Unpacked Size

73 kB

Total Files


Last publish


  • marchaos