计时器模拟

The native timer functions (i.e., setTimeout(), setInterval(), clearTimeout(), clearInterval()) are less than ideal for a testing environment since they depend on real time to elapse. Jest可以通过一个函数转换计时器以便允许你控制时间流量。 Great Scott!

info

Also see Fake Timers API documentation.

Enable Fake Timers

In the following example we enable fake timers by calling jest.useFakeTimers(). This is replacing the original implementation of setTimeout() and other timer functions. 计时器可以恢复他们默认的行为通过jest.useRealTimers().

timerGame.js

function timerGame(callback) {
  console.log('Ready....go!');
  setTimeout(() => {
    console.log("Time's up -- stop!");
    callback && callback();
  }, 1000);
}

module.exports = timerGame;

__tests__/timerGame-test.js

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

test('waits 1 second before ending the game', () => {
  const timerGame = require('../timerGame');
  timerGame();

  expect(setTimeout).toHaveBeenCalledTimes(1);
  expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
});

运行所有计时器

对于这个模块我们还需要写一个测试，用于判断回调函数是否在1秒后被调用的。 为此，我们将使用Jest的定时器控制API，用于在测试中将时间“快进”到正确的时间点。

jest.useFakeTimers();
test('calls the callback after 1 second', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);

  // At this point in time, the callback should not have been called yet
  expect(callback).not.toBeCalled();

  // Fast-forward until all timers have been executed
  jest.runAllTimers();

  // Now our callback should have been called!
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

运行等待计时器

There are also scenarios where you might have a recursive timer – that is a timer that sets a new timer in its own callback. For these, running all the timers would be an endless loop, throwing the following error: "Aborting after running 100000 timers, assuming an infinite loop!"

If that is your case, using jest.runOnlyPendingTimers() will solve the problem:

infiniteTimerGame.js

function infiniteTimerGame(callback) {
  console.log('Ready....go!');

  setTimeout(() => {
    console.log("Time's up! expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

__tests__/infiniteTimerGame-test.js

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

describe('infiniteTimerGame', () => {
  test('schedules a 10-second timer after 1 second', () => {
    const infiniteTimerGame = require('../infiniteTimerGame');
    const callback = jest.fn();

    infiniteTimerGame(callback);

    // At this point in time, there should have been a single call to
    // setTimeout to schedule the end of the game in 1 second.
    expect(setTimeout).toHaveBeenCalledTimes(1);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);

    // Fast forward and exhaust only currently pending timers
    // (but not any new timers that get created during that process)
    jest.runOnlyPendingTimers();

    // At this point, our 1-second timer should have fired its callback
    expect(callback).toBeCalled();

    // And it should have created a new timer to start the game over in
    // 10 seconds
    expect(setTimeout).toHaveBeenCalledTimes(2);
    expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 10000);
  });
});

note

For debugging or any other reason you can change the limit of timers that will be run before throwing an error:

jest.useFakeTimers({timerLimit: 100});

使用时间的高级计时器

另一种可选方式是使用 jeste. advancertimersbytime (msToRun)。 通过调用msToRun这个 API时， 所有计时器都将以毫秒增长。 When this API is called, all timers are advanced by msToRun milliseconds. 所有通过setTimeout() 或setInterval() 而处于任务队列中等待中的“宏任务”和一切其他应该在本时间片中被执行的东西都应该被执行。 Additionally, if those macro-tasks schedule new macro-tasks that would be executed within the same time frame, those will be executed until there are no more macro-tasks remaining in the queue that should be run within msToRun milliseconds.

timerGame.js

function timerGame(callback) {
  console.log('Ready....go!');
  setTimeout(() => {
    console.log("Time's up -- stop!");
    callback && callback();
  }, 1000);
}

module.exports = timerGame;

__tests__/timerGame-test.js

jest.useFakeTimers();
it('calls the callback after 1 second via advanceTimersByTime', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);

  // At this point in time, the callback should not have been called yet
  expect(callback).not.toBeCalled();

  // Fast-forward until all timers have been executed
  jest.advanceTimersByTime(1000);

  // Now our callback should have been called!
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

最后，在某些测试中你可能需要清除所有等待状态下的定时器，为此，可以使用 jest.clearAllTimers()。 For this, we have jest.clearAllTimers().

Selective Faking

Sometimes your code may require to avoid overwriting the original implementation of one or another API. If that is the case, you can use doNotFake option. For example, here is how you could provide a custom mock function for performance.mark() in jsdom environment:

/**
 * @jest-environment jsdom
 */

const mockPerformanceMark = jest.fn();
window.performance.mark = mockPerformanceMark;

test('allows mocking `performance.mark()`', () => {
  jest.useFakeTimers({doNotFake: ['performance']});

  expect(window.performance.mark).toBe(mockPerformanceMark);
});