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Version: 26.x

Timer Mocks

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 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);
});

Здесь мы включим поддельные таймеры вызовом jest.useFakeTimers();. Эта опция блокирует setTimeout и другие функции таймера с mock-функций. Если вы запускаете несколько тестов внутри одного файла или описываете блок, jest.useFakeTimers(); можно вызывать перед каждым тестом вручную или с помощью функции настройки, такой как beforeEach beforeEach. Если это не произойдет, то не будет сброшен внутренний счетчик использования.

Запуск всех таймеров#

Another test we might want to write for this module is one that asserts that the callback is called after 1 second. To do this, we're going to use Jest's timer control APIs to fast-forward time right in the middle of the test:

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);
});

Запуск отложенных таймеров#

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… so something like jest.runAllTimers() is not desirable. 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#

переименован из runTimersToTime в advanceTimersByTime в Jest 22.0.0#

Another possibility is use jest.advanceTimersByTime(msToRun). When this API is called, all timers are advanced by msToRun milliseconds. 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 time frame, will be executed.

// 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);
// 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);
});

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().

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