jest-async.test.js

const incrementPromise = (value) => {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(value + 1);
    }, 300);
  });
};

describe('incrementPromise', () => {
  beforeEach(() => {
    jest.useFakeTimers();
  });

  afterEach(() => {
    jest.useRealTimers();
  });

  it('should increment the number after 300 ms', () => {
    // creates a Promise of "pending" status
    const promise = incrementPromise(3);

    // advance the Jest system clock by 300ms
    jest.advanceTimersByTime(300);

    // return the promise, and setup the expectation in the .then block
    return promise.then((value) => {
      expect(value).toBe(4);
    });
  });
});