Mock dependency in Jest with TypeScript

Question

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 from ts-jest like explained here

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

// here the whole foo var is mocked deeply
const mockedFoo = 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(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')