Typescript and Jest: Avoiding type errors on mocked functions

Issue

When wanting to mock external modules with Jest, we can use the jest.mock() method to auto-mock functions on a module.

We can then manipulate and interrogate the mocked functions on our mocked module as we wish.

For example, consider the following contrived example for mocking the axios module:

import myModuleThatCallsAxios from '../myModule';
import axios from 'axios';

jest.mock('axios');

it('Calls the GET method as expected', async () => {
  const expectedResult: string = 'result';

  axios.get.mockReturnValueOnce({ data: expectedResult });
  const result = await myModuleThatCallsAxios.makeGetRequest();

  expect(axios.get).toHaveBeenCalled();
  expect(result).toBe(expectedResult);
});

The above will run fine in Jest but will throw a Typescript error:

Property 'mockReturnValueOnce' does not exist on type '(url: string, config?: AxiosRequestConfig | undefined) => AxiosPromise'.

The typedef for axios.get rightly doesn't include a mockReturnValueOnce property. We can force Typescript to treat axios.get as an Object literal by wrapping it as Object(axios.get), but:

What is the idiomatic way to mock functions while maintaining type safety?

Solution

Starting with ts-jest 27.0 mocked from ts-jest will be deprecated and removed in 28.0 you can check it in the official documentation. So please use instead mocked from jest. Here's the documentation

So for your example:

import myModuleThatCallsAxios from '../myModule';
import axios from 'axios';

jest.mock('axios');

// OPTION - 1
const mockedAxios = jest.mocked(axios, true)
// your original `it` block
it('Calls the GET method as expected', async () => {
  const expectedResult: string = 'result';

  mockedAxios.mockReturnValueOnce({ data: expectedResult });
  const result = await myModuleThatCallsAxios.makeGetRequest();

  expect(mockedAxios.get).toHaveBeenCalled();
  expect(result).toBe(expectedResult);
});

Answered By - Melchia