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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 pode trocar temporizadores por funções que permitem controlar a passagem do tempo. 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('espera 1 segundo antes de terminar o jogo', () => {
  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.

Executar todos os temporizadores

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('chama a callback depois de 1 segundo', () => {
  const timerGame = require('../timerGame');
  const callback = jest.fn();

  timerGame(callback);

  // Neste ponto no tempo, a callback não deveria ter sido chamada ainda
  expect(callback).not.toBeCalled();

  // Avançando até que todos os temporizadores tenham executado
  jest.runAllTimers();

  // Agora nossa callback deve ter sido chamada!
  expect(callback).toBeCalled();
  expect(callback).toHaveBeenCalledTimes(1);
});

Executar Temporizadores Pendentes

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. Para esses casos, você pode usar 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);
  });
});

Avance Temporizadores por Tempo

renomeado de runTimersToTime para advanceTimersByTime no Jest 22.0.0

Outra possibilidade é usar o 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.