Mock dependency in Jest with TypeScript

When testing a module that has a dependency in a different file and assigning that module to be a jest.mock, TypeScript gives an error that the method mockReturnThisOnce (or any other jest.mock method) does not exist on the dependency, this is because it is previously typed.

What is the proper way to get TypeScript to inherit the types from jest.mock?

Here is a quick example.

Dependency

const myDep = (name: string) => name;
export default myDep;

test.ts

import * as dep from '../depenendency';
jest.mock('../dependency');

it('should do what I need', () => {
  //this throws ts error
  // Property mockReturnValueOnce does not exist on type (name: string)....
  dep.default.mockReturnValueOnce('return')
}

I feel like this is a very common use case and not sure how to properly type this.

You can use type casting and your test.ts should look like this:

import * as dep from '../dependency';
jest.mock('../dependency');

const mockedDependency = <jest.Mock<typeof dep.default>>dep.default;

it('should do what I need', () => {
  //this throws ts error
  // Property mockReturnValueOnce does not exist on type (name: string)....
  mockedDependency.mockReturnValueOnce('return');
});

TS transpiler is not aware that jest.mock('../dependency'); changes type of dep thus you have to use type casting. As imported dep is not a type definition you have to get its type with typeof dep.default.

Here are some other useful patterns I've found during my work with Jest and TS

When imported element is a class then you don't have to use typeof for example:

import { SomeClass } from './SomeClass';

jest.mock('./SomeClass');

const mockedClass = <jest.Mock<SomeClass>>SomeClass;

This solution is also useful when you have to mock some node native modules:

import { existsSync } from 'fs';

jest.mock('fs');

const mockedExistsSync = <jest.Mock<typeof existsSync>>existsSync;

In case you don't want to use jest automatic mock and prefer create manual one

import TestedClass from './TestedClass';
import TestedClassDependency from './TestedClassDependency';

const testedClassDependencyMock = jest.fn<TestedClassDependency>(() => ({
  // implementation
}));

it('Should throw an error when calling playSomethingCool', () => {
  const testedClass = new TestedClass(testedClassDependencyMock());
});

testedClassDependencyMock() creates mocked object instance TestedClassDependency can be either class or type or interface

Use the mocked helper like explained here

// foo.spec.ts
import { foo } from './foo'
jest.mock('./foo')

// here the whole foo var is mocked deeply
const mockedFoo = jest.mocked(foo, true)

test('deep', () => {
  // there will be no TS error here, and you'll have completion in modern IDEs
  mockedFoo.a.b.c.hello('me')
  // same here
  expect(mockedFoo.a.b.c.hello.mock.calls).toHaveLength(1)
})

test('direct', () => {
  foo.name()
  // here only foo.name is mocked (or its methods if it's an object)
  expect(jest.mocked(foo.name).mock.calls).toHaveLength(1)
})

There are two solutions tested for TypeScript version 3.x and 4.x, both are casting desired function

1) Use jest.MockedFunction

import * as dep from './dependency';

jest.mock('./dependency');

const mockMyFunction = dep.myFunction as jest.MockedFunction<typeof dep.myFunction>;

2) Use jest.Mock

import * as dep from './dependency';

jest.mock('./dependency');

const mockMyFunction = dep.default as jest.Mock;

---

There is no difference between these two solutions. The second one is shorter and I would therefore suggest using that one.

Both casting solutions allows to call any jest mock function on mockMyFunction like mockReturnValue or mockResolvedValue https://jestjs.io/docs/en/mock-function-api.html

mockMyFunction.mockReturnValue('value');

mockMyFunction can be used normally for expect

expect(mockMyFunction).toHaveBeenCalledTimes(1);

I use the pattern from @types/jest/index.d.ts just above the type def for Mocked (line 515):

import { Api } from "../api";
jest.mock("../api");

const myApi: jest.Mocked<Api> = new Api() as any;
myApi.myApiMethod.mockImplementation(() => "test");

Cast as jest.Mock

Simply casting the function to jest.Mock should do the trick:

(dep.default as jest.Mock).mockReturnValueOnce('return')

Use as jest.Mock and nothing else

The most concise way of mocking a module exported as default in ts-jest that I can think of really boils down to casting the module as jest.Mock.

Code:

import myDep from '../dependency' // No `* as` here

jest.mock('../dependency')

it('does what I need', () => {
  // Only diff with pure JavaScript is the presence of `as jest.Mock`
  (myDep as jest.Mock).mockReturnValueOnce('return')

  // Call function that calls the mocked module here

  // Notice there's no reference to `.default` below
  expect(myDep).toHaveBeenCalled()
})

Benefits:

• does not require referring to the default property anywhere in the test code - you reference the actual exported function name instead,
• you can use the same technique for mocking named exports,
• no * as in the import statement,
• no complex casting using the typeof keyword,
• no extra dependencies like mocked.

Here's what I did with [email protected] and [email protected]:

source:

class OAuth {

  static isLogIn() {
    // return true/false;
  }

  static getOAuthService() {
    // ...
  }
}

test:

import { OAuth } from '../src/to/the/OAuth'

jest.mock('../src/utils/OAuth', () => ({
  OAuth: class {
    public static getOAuthService() {
      return {
        getAuthorizationUrl() {
          return '';
        }
      };
    }
  }
}));

describe('createMeeting', () => {
  test('should call conferenceLoginBuild when not login', () => {
    OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
      return false;
    });

    // Other tests
  });
});

This is how to mock a non-default class and it's static methods:

jest.mock('../src/to/the/OAuth', () => ({
  OAuth: class {
    public static getOAuthService() {
      return {
        getAuthorizationUrl() {
          return '';
        }
      };
    }
  }
}));

Here should be some type conversion from the type of your class to jest.MockedClass or something like that. But it always ends up with errors. So I just used it directly, and it worked.

test('Some test', () => {
  OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
    return false;
  });
});

But, if it's a function, you can mock it and do the type conversation.

jest.mock('../src/to/the/Conference', () => ({
  conferenceSuccessDataBuild: jest.fn(),
  conferenceLoginBuild: jest.fn()
}));
const mockedConferenceLoginBuild = conferenceLoginBuild as 
jest.MockedFunction<
  typeof conferenceLoginBuild
>;
const mockedConferenceSuccessDataBuild = conferenceSuccessDataBuild as 
jest.MockedFunction<
  typeof conferenceSuccessDataBuild
>;

I have found this in @types/jest:

/**
  * Wrap a function with mock definitions
  *
  * @example
  *
  *  import { myFunction } from "./library";
  *  jest.mock("./library");
  *
  *  const mockMyFunction = myFunction as jest.MockedFunction<typeof myFunction>;
  *  expect(mockMyFunction.mock.calls[0][0]).toBe(42);
*/

Note: When you do const mockMyFunction = myFunction and then something like mockFunction.mockReturnValue('foo'), you're a changing myFunction as well.

Source: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L1089

As of Jest 24.9.0 here is how you can mock and correctly type both your Class/Object/function and Jest properties.

jest.MockedFunction

jest.MockedClass

What we would like for a typed mock is that the mocked object type contains the union of the mocked object type and the type of Jest mocks.

import foo from 'foo';
jest.mock('foo');

const mockedFoo = foo as jest.MockedFunction<typeof foo>;
// or: const mockedFooClass = foo as jest.MockedClass<typeof FooClass>;


mockedFoo.mockResolvedValue('mockResult');

// Or:
(mockedFoo.getSomething as jest.MockedFunction<typeof mockedFoo.getSomething>).mockResolvedValue('mockResult');

As you can see, you can either manually cast what you need or you'll need something to traverse all foo's properties/methods to type/cast everything.

To do that (deep mock types) you can use jest.mocked() introduced in Jest 27.4.0

import foo from 'foo';
jest.mock('foo');

const mockedFoo = jest.mocked(foo, true); 

mockedFoo.mockImplementation() // correctly typed
mockedFoo.getSomething.mockImplementation() // also correctly typed

The top rated solution by Artur Górski does not work with the last TS and Jest. Use MockedClass

import SoundPlayer from '../sound-player';

jest.mock('../sound-player'); // SoundPlayer is now a mock constructor

const SoundPlayerMock = SoundPlayer as jest.MockedClass<typeof SoundPlayer>;

The latest jest allows you to do this very easily with jest.mocked

import * as dep from '../dependency';

jest.mock('../dependency');

const mockedDependency = jest.mocked(dep);

it('should do what I need', () => {
  mockedDependency.mockReturnValueOnce('return');
});

This is ugly, and in fact getting away from this ugliness is why I even looked at this question, but to get strong typing from a module mock, you can do something like this:

const myDep = (require('./dependency') as import('./__mocks__/dependency')).default;

jest.mock('./dependency');

Make sure you require './dependency' rather than the mock directly, or you will get two different instantiations.

For me this was enough:

let itemQ: queueItemType
jest.mock('../dependency/queue', () => {
    return {
        add: async (item: queueItemType, ..._args: any) => {
            // then we can use the item that would be pushed to the queue in our tests
            itemQ = item
            return new Promise(resolve => {
                resolve('Mocked')
            })
        },
    }
})

Then, whenever the add method is called it will execute this code above instead of pushing it to the queue, in this case.

A recent library solves this problem with a babel plugin: https://github.com/userlike/joke

Example:

import { mock, mockSome } from 'userlike/joke';

const dep = mock(import('./dependency'));

// You can partially mock a module too, completely typesafe!
// thisIsAMock has mock related methods
// thisIsReal does not have mock related methods
const { thisIsAMock, thisIsReal } = mockSome(import('./dependency2'), () => ({ 
  thisIsAMock: jest.fn() 
}));

it('should do what I need', () => {
  dep.mockReturnValueOnce('return');
}

Be aware that dep and mockReturnValueOnce are fully type safe. On top, tsserver is aware that depencency was imported and was assigned to dep so all automatic refactorings that tsserver supports will work too.

Note: I maintain the library.