Async Jest

.resolves and .rejects

By using .resolves matcher, Jest will wait for that Promise to resolve.

• If the Promise is rejected, it'll automatically fail.

// resolves
test('testing Promise return expected resolved value', async () => {
  await expect( async () => await service.fetchData()).resolves.toBe('value');
});

// rejects
test('testing Promise rejects to throw an expected error', async () => {
  await expect( async () => await service.fetchData()).rejects.toThrow(new NotFoundError().message);
});

Mocking Promise

Implementation

// myModule.js

// Function that returns a Promise
export const fetchData = () => {
  return new Promise((resolve, reject) => {
    // Some asynchronous operation
    setTimeout(() => {
      resolve('Mocked Data');
    }, 1000);
  });
};

Testing for resolved result:

import { fetchData } from './myModule';

jest.mock('./myModule'); // Automatically mocks the entire module

test('should mock the return value of fetchData', async () => {
  // Set the mocked return value
  fetchData.mockResolvedValue('Mocked Data');

  // Your test code that uses fetchData
  const result = await fetchData();

  // Assertions
  expect(result).toBe('Mocked Data');
});

Alternatively, testing for Promise value:

(fetchData as jest.Mock).mockReturnValue(Promise.resolve())
(fetchData as jest.Mock).mockReturnValue(Promise.reject())