How to mock const method in jest?

Question

I unit test code in typescript, use jest. Please teach me how to mock getData to return the expected value. My code as below:

// File util.ts
export const getData = async () => {
    // Todo something
    return data;
}

// File execution.ts import { getData } from './util';
function execute()
{
    // todo something
    const data = await getData();
    // todo something 
}

Answer 1

The problem is that your function returns a promise. Depends on how you use it there are several ways to mock it.

The simplest way would be to mock it directly, but then it will always return the same value:

// note, the path is relative to your test file
jest.mock('./util', () => ({ getData: () => 'someValue' }));

If you want to test both the resolved and the rejected case you need to mock getData so it will return a spy where you later on can change the implementation use mockImplementation. You also need to use async/await to make the test work, have a look at the docs about asynchronous testing:

import { getData } from './util';
jest.mock('./util', () => ({ getData: ()=> jest.fn() }));

it('success case', async () => {
  const result = Promise.resolve('someValue');
  getData.mockImplementation(() => result);

  // call your function to test
  await result; // you need to use await to make jest aware of the promise
});

it('error case', async () => {
  const result = Promise.reject(new Error('someError'));
  getData.mockImplementation(() => result);

  // call your function to test
  await expect(result).rejects.toThrow('someError');
});

Answer 2

Try the following in your test file. Import the function from the module.

 import { getData } from './util';

Then mock the module with the function and its return value after all the import statements

jest.mock('./util', () => ({ getData: jest.fn() }))
getData.mockReturnValue("abc");

Then use it in your tests.