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Version: 26.x

Dubluri de timp

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 poate înlocui cronometrele cu funcţii care vă permit să controlaţi trecerea timpului. 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);
});

Here we enable fake timers by calling jest.useFakeTimers();. This mocks out setTimeout and other timer functions with mock functions. If running multiple tests inside of one file or describe block, jest.useFakeTimers(); can be called before each test manually or with a setup function such as beforeEach. Not doing so will result in the internal usage counter not being reset.

Rularea tuturor cronometrelor#

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);
});

Rularea cronometrelor în așteptare#

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. Pentru aceste cazuri s-ar putea folosi 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 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);
});
});

Advance Timers by Time#

renamed from runTimersToTime to advanceTimersByTime in Jest 22.0.0#

Another possibility is use jest.advanceTimersByTime(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 time frame, 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);
// 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.