Working with Timer in Jest

Since setTimeout(), setInterval(), clearTimeout(), clearInterval() are not ideal for testing environment because they are depended on real time to elapse. But Jest comes with some handy methods to do that.

1. Fake timers

By calling jest.useFakeTimers(), Jest will replace the original implementation of the above functions.

// timer.ts

const timerExample = (timeout: number, callback: (T?: any) => void) => {
  setTimeout(() => {
    callback();
  }, timeout);
};

// timer.test.ts

jest.useFakeTimers();
jest.spyOn(global, "setTimeout");

import { timerExample } from "./../src/examples/timer";

describe("timer", () => {
  it("testing timer", () => {
    timerExample(100, () => {});
    expect(setTimeout).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 100);
  });
});

We can stimulate the timing by using:

• jest.runAllTimers(): will fast-forward until all timers have been executed.
• jest.advanceTimersByTime(msToRun): all timers are advanced by msToRun milliseconds.
  • required to use jest.useFakeTimers();
  • all “macro-tasks” that have been queued via setTimeout() or setInterval(), and would be executed during this time frame, will be executed.

// timer.test.ts

describe("timer", () => {
  it("testing timer", () => {
    const callback = jest.fn();

    timerExample(100, callback);

    expect(callback).not.toHaveBeenCalledTimes(1);

    jest.runAllTimers();

    expect(callback).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 100);
  });
});

2. Spy on the global Date

Sometimes, we might need to use:

jest.spyOn(global.Date, "now").mockReturnValueOnce(Date.now() + 1000);

to make the time relate to Date.

it("should calculate duration correctly", async () => {
  videoRecording.start();
  jest.spyOn(global.Date, "now").mockReturnValueOnce(Date.now() + 2000);
  videoRecording.stop();
  expect(videoRecording.duration).toBe(2000); // Duration should be 2 seconds
  videoRecording.reset();
});

Refs:

https://jestjs.io/docs/timer-mocks