JestMocks.js

export function fullName(firstName, lastName, callback) {
  callback(`${firstName} ${lastName}`);
}

export function promiseExample() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(true);
    }, 100);
  });
}
/* Tests */

test('callback funct gets called', () => {
  const mockedCallback = jest.fn();
  fullName('Trusting', 'Social', mockedCallback);
  expect(mockedCallback).toHaveBeenCalled();
});

test('callback funct mock implementation', () => {
  const mockedCallback = jest.fn();
  mockedCallback.mockImplementation((fullName) => console.log(`Hello ${fullName}`);
  fullName('Trusting', 'Social', mockedCallback);
  expect(mockedCallback).toHaveBeenCalledWith('Trusting Social');
});

test('mock implementation with return value', () => {
  const mockedCallback = jest.fn();
  mockedCallback.mockReturnValue('Hello Mocked Return Value');

  expect(fullName('Trusting', 'Social')).toBe('Hello Mocked Return Value');
  expect(mockedCallback).toHaveBeenCalledWith('Trusting Social');
});

test('async await with jest', async () => {
  const promiseResolvedValue = await promiseExample();
  expect(promiseResolvedValue).toBeTruthy();
});