原生的定时器函数(如：setTimeout, setInterval, clearTimeout, clearInterval)并不是很方便测试，因为程序需要等待相应的延时。 Jest can swap out timers with functions that allow you to control the passage of time. Great Scott!

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

// __tests__/timerGame-test.js
'use strict';
jest.useFakeTimers();
test('waits 1 second before ending the game', () => {
  const timerGame = require('../timerGame');
  timerGame();
  expect(setTimeout).toHaveBeenCalledTimes(1);
  expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
});

在这里我们通过jest.useFakeTimers();来模拟定时器函数。 通过mock函数可以模拟setTimeout和其他的定时器函数。

Run All Timers

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。

test('calls the callback after 1 second', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();
  timerGame(callback);
  // 在这个时间点，定时器的回调不应该被执行
  expect(callback).not.toBeCalled();
  // “快进”时间使得所有定时器回调被执行
  jest.runAllTimers();
  // 现在回调函数应该被调用了！
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

Run Pending Timers

在某些场景下你可能还需要“循环定时器”——在定时器的callback函数中再次设置一个新定时器。 对于这种情况，如果将定时器一直运行下去那将陷入死循环，所以在此场景下不应该使用jest.runAllTimers() For these cases you might use jest.runOnlyPendingTimers():

// infiniteTimerGame.js
'use strict';
function infiniteTimerGame(callback) {
  console.log('Ready....go!');
  setTimeout(() => {
    console.log("Time's up! 10 seconds before the next game starts...");
    callback && callback();
    // Schedule the next game in 10 seconds
    setTimeout(() => {
      infiniteTimerGame(callback);
    }, 10000);
  }, 1000);
}
module.exports = infiniteTimerGame;

// __tests__/infiniteTimerGame-test.js
'use strict';
jest.useFakeTimers();
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 it's 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);
  });
});

Advance Timers by Time

从22.0.0版本的Jest开始，runTimersToTime被重命名为advanceTimersByTime。

另一种可选方式是使用 jeste. advancertimersbytime (msToRun)。 When this API is called, all timers are advanced by msToRun milliseconds. All pending "macro-tasks" that have been queued via setTimeout() or setInterval(), and would be executed during this timeframe, will be executed. 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
'use strict';
function timerGame(callback) {
  console.log('Ready....go!');
  setTimeout(() => {
    console.log("Time's up -- stop!");
    callback && callback();
  }, 1000);
}
module.exports = timerGame;

it('calls the callback after 1 second via advanceTimersByTime', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();
  timerGame(callback);
  // 在这个时间点，回调函数不应该被执行
  expect(callback).not.toBeCalled();
  // “快进”时间，使得所有定时器回调都被执行
  jest.advanceTimersByTime(1000);
  // 到这里，所有的定时器回调都应该被执行了！
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

Lastly, it may occasionally be useful in some tests to be able to clear all of the pending timers. For this, we have jest.clearAllTimers().

这个例子的代码位于:examples/timer。