计时器模拟

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!

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(); // or you can set "timers": "fake" globally in configuration file

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。 如果你需要在一个文件或一个describe块中运行多次测试，可以在每次测试前手动添加jest.useFakeTimers();，或者在beforeEach中添加。 如果不这样做的话将导致内部的定时器不被重置。

All of the following functions need fake timers to be set, either by jest.useFakeTimers() or via "timers": "fake" in the config file.

Currently, two implementations of the fake timers are included - modern and legacy, where legacy is still the default one. See configuration for how to configure it.

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

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。 For these, running all the timers would be an endless loop, throwing the following error:

Ran 100000 timers, and there are still more! Assuming we've hit an infinite recursion and bailing out...

So something like jest.runAllTimers() is not desirable. 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! expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

__tests__/infiniteTimerGame-test.js

'use strict';

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);
  });
});

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

'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

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().

The code for this example is available at examples/timer.