How to change mock implementation on a per single test basis [Jestjs]

Question

I\'d like to change the implementation of a mocked dependency on a per single test basis by extending the default mock\'s

Answer 1

Vanilla JS

Use mockFn.mockImplementation(fn).

import { funcToMock } from './somewhere';
jest.mock('./somewhere');

beforeEach(() => {
  funcToMock.mockImplementation(() => { /* default implementation */ });
});

test('case that needs a different implementation of funcToMock', () => {
  funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
  // ...
});

TypeScript

In addition to the Vanilla JS solution:

To prevent the message mockImplementation is not a property of funcToMock, you will need to specify the type, e.g. by changing the top line from above to the following:

import { (funcToMock as jest.Mock) } from './somewhere';

A question addressing this issue can be found here: jest typescript property mock does not exist on type