Use mockFn.mockImplementation(fn).
import { funcToMock } from './somewhere';
jest.mock('./somewhere');
beforeEach(() => {
funcToMock.mockImplementation(() => { /* default implementation */ });
// (funcToMock as jest.Mock)... in TS
});
test('case that needs a different implementation of funcToMock', () => {
funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
// (funcToMock as jest.Mock)... in TS
// ...
});