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Timer Mocks

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('Times 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 timerG