Unable to mock startOfToday from "date-fns"

Question

I'm trying to mock a function from js date-fns module.

Top of my test:

import { startOfToday } from "date-fns"

During my test I try to mock:

startOfToday = jest.fn(() => 'Tues Dec 28 2019 00:00:00 GMT+0000')

Which gives me the error when I run the tests:

"startOfToday" is read-only

Answer 1

You can use jest.mock(moduleName, factory, options) method to mock date-fns module.

E.g.

index.js:

import { startOfToday } from 'date-fns';

export default function main() {
  return startOfToday();
}

index.spec.js:

import main from './';
import { startOfToday } from 'date-fns';

jest.mock('date-fns', () => ({ startOfToday: jest.fn() }));

describe('59515767', () => {
  afterEach(() => {
    jest.resetAllMocks();
  });
  it('should pass', () => {
    startOfToday.mockReturnValueOnce('Tues Dec 28 2019 00:00:00 GMT+0000');
    const actual = main();
    expect(actual).toBe('Tues Dec 28 2019 00:00:00 GMT+0000');
  });
});

Unit test result:

 PASS  src/stackoverflow/59515767/index.spec.js (10.095s)
  59515767
    ✓ should pass (5ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.js |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        12.081s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/59515767