jest typescript - Mock Date constructor

问题

I'm trying to mock new Date() to return a specific date. The following code:

const now = new Date()
jest.spyOn(global, 'Date').mockImplementation(() => now)

gives a compilation error: Argument of type '() => Date' is not assignable to parameter of type '() => string'. Type 'Date' is not assignable to type 'string'.

I think the reason is that jest thinks I'm trying to mock Date() instead of new Date(). Indeed, Date() returns a string. How can I solve this issue?

回答1:

A workaround is to use the mockdate library, which can be used to change when "now" is.

const MockDate = require('mockdate');

test('Mock Date to change when "now" is', () => {
  console.log('Normal:   ', new Date().getTime());

  MockDate.set(new Date(1466424490000));

  console.log('Mocked:   ', new Date().getTime());

  MockDate.reset();

  console.log('Restored: ', new Date().getTime());
});

And the test result is like:

$ npm run test
> jest

 PASS  src/test.ts
  ✓ Mock Date to change when "now" is (8ms)

  console.log src/test.ts:4
    Normal:    1585505136315

  console.log src/test.ts:8
    Mocked:    1466424490000

  console.log src/test.ts:12
    Restored:  1585505136322

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        1.804s

See the reference project on GitHub.

来源：https://stackoverflow.com/questions/60912023/jest-typescript-mock-date-constructor