Async unit tests with Jest

example.spec.js

const INTERVAL = 100;

function myFunction () {
  setTimeout(() => {
    console.log('hi');
  }, INTERVAL);
}

describe('myFunction', () => {
  it('should be testable', () => {
    // spy on console.log
    console.log = jest.fn();
    
    // instructs Jest to fail the test if the number of `expect` invocations in the
    // block beneath this expression does not equal the argument provided
    expect.assertions(1);
    
    // adding a return statement with a Promise tells Jest not to terminate this spec
    // until the Promise is resolved or rejected
    return new Promise((res) => {
      myFunction();
      // JavaScript's clock isn't 100% precise, let's give 50ms of wiggle room
      setTimeout(res, INTERVAL + 50);
    })
      .then(() => {
        expect(console.log).toHaveBeenCalledWith('hi');
      });
  });
});