How to change mock implementation on a per single test basis with Jest.js and JavaScript?

In Jest, you can change the mock implementation on a per-test basis using jest.fn() and providing a new implementation as the first argument in your test case.

Here’s an example:

const myModule = require('./myModule');

describe('myModule', () => {
  it('uses the default implementation', () => {
    expect(myModule.getData()).toBe('default');
  });

  it('uses a custom implementation', () => {
    myModule.getData = jest.fn(() => 'custom');
    expect(myModule.getData()).toBe('custom');
  });
});

In the first test case, the default implementation is used, while in the second test case, the mock function is replaced with a new implementation. Note that you can only replace the mock implementation within the scope of the test case, and it won’t affect other test cases.