foo-test.js

// __tests__/Foo-test.js

describe('hai', function() {
  it('asdf', function() {
    var mockedImplWithReturn = jest.genMockFn().mockImpl(function() {
      console.log('  running impl');
      return 'impl value';
    }).mockReturnValue('mock return value');

    var mockedImplWithoutReturn = jest.genMockFn().mockImpl(function() {
      console.log('  running impl');
      return 'impl value';
    });

    var ret;

    console.log('Running mockedImplWithReturn:');
    ret = mockedImplWithReturn();
    console.log('  Returned:', ret);

    console.log('Running mockedImplWithoutReturn:');
    ret = mockedImplWithoutReturn();
    console.log('  Returned:', ret);
  });
});

gistfile1.txt

Found 1 matching tests...
 PASS  __tests__/foo-test.js (0.011s)
Running mockedImplWithReturn:
  Returned: mock return value
Running mockedImplWithoutReturn:
  running impl
  Returned: impl value
1 tests passed (1 total)
Run time: 1.075s