计时器模拟

    info

    Also see Fake Timers API documentation.

    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

    __tests__/timerGame-test.js

    1. jest.useFakeTimers();
    2. jest.spyOn(global, 'setTimeout');
    4. test('waits 1 second before ending the game', () => {
    5.   const timerGame = require('../timerGame');
    6.   timerGame();
    8.   expect(setTimeout).toHaveBeenCalledTimes(1);
    9.   expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
    10. });
    1. jest.useFakeTimers();
    2. test('calls the callback after 1 second', () => {
    3.   const timerGame = require('../timerGame');
    4.   const callback = jest.fn();
    6.   timerGame(callback);
    8.   // At this point in time, the callback should not have been called yet
    9.   expect(callback).not.toBeCalled();
    11.   // Fast-forward until all timers have been executed
    12.   jest.runAllTimers();
    14.   // Now our callback should have been called!
    15.   expect(callback).toHaveBeenCalledTimes(1);

    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

    __tests__/infiniteTimerGame-test.js

    1. jest.useFakeTimers();
    2. jest.spyOn(global, 'setTimeout');
    4. describe('infiniteTimerGame', () => {
    5.   test('schedules a 10-second timer after 1 second', () => {
    6.     const infiniteTimerGame = require('../infiniteTimerGame');
    7.     const callback = jest.fn();
    9.     infiniteTimerGame(callback);
    11.     // At this point in time, there should have been a single call to
    12.     // setTimeout to schedule the end of the game in 1 second.
    13.     expect(setTimeout).toHaveBeenCalledTimes(1);
    14.     expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 1000);
    16.     // Fast forward and exhaust only currently pending timers
    17.     // (but not any new timers that get created during that process)
    18.     jest.runOnlyPendingTimers();
    20.     // At this point, our 1-second timer should have fired its callback
    21.     expect(callback).toBeCalled();
    23.     // And it should have created a new timer to start the game over in
    24.     // 10 seconds
    25.     expect(setTimeout).toHaveBeenCalledTimes(2);
    26.     expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 10000);
    27.   });

    计时器模拟 - 图2note

    1. jest.useFakeTimers({timerLimit: 100});

    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 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

    __tests__/timerGame-test.js

    1. jest.useFakeTimers();
    2. it('calls the callback after 1 second via advanceTimersByTime', () => {
    3.   const timerGame = require('../timerGame');
    4.   const callback = jest.fn();
    6.   timerGame(callback);
    8.   // At this point in time, the callback should not have been called yet
    9.   expect(callback).not.toBeCalled();
    11.   // Fast-forward until all timers have been executed
    12.   jest.advanceTimersByTime(1000);
    14.   // Now our callback should have been called!
    15.   expect(callback).toBeCalled();
    16.   expect(callback).toHaveBeenCalledTimes(1);
    17. });

    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().

    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:

    1. /**
    2.  * @jest-environment jsdom
    3.  */
    5. const mockPerformanceMark = jest.fn();
    6. window.performance.mark = mockPerformanceMark;
    8. test('allows mocking `performance.mark()`', () => {
    9.   jest.useFakeTimers({doNotFake: ['performance']});
    11.   expect(window.performance.mark).toBe(mockPerformanceMark);