36. create a fake timer(setTimeout)

setTimeout adds task in to a task queue to be handled later, the time actually is no accurate. (Event Loop).

This is OK in general web application, but might be problematic in test.

For example, at 5. implement throttle() with leading & trailing option we need to test the timer with more accurate approach.

Could you implement your own setTimeout() and clearTimeout() to be sync? so that they have accurate timing for test. This is what FakeTimes are for.

By "accurate", it means suppose all functions cost no time, we start our function at time 0, then setTimeout(func1, 100) would schedule func1 exactly at 100.

You need to replace Date.now() as well to provide the time.

class FakeTimer {
  install() {
    // setTimeout(), clearTimeout(), and Date.now() 
    // are replaced
  }

  uninstall() {
    // restore the original APIs
    // setTimeout(), clearTimeout() and Date.now()
  }

  tick() {
     // run all the schedule functions in order
  }
}

Your code is tested like this

const fakeTimer = new FakeTimer()
fakeTimer.install()

const logs = []
const log = (arg) => {
   logs.push([Date.now(), arg])
}

setTimeout(() => log('A'), 100)
// log 'A' at 100

const b = setTimeout(() => log('B'), 110)
clearTimeout(b)
// b is set but cleared

setTimeout(() => log('C'), 200)

expect(logs).toEqual([[100, 'A'], [200, 'C']])

fakeTimer.uninstall()

Note

Only Date.now() is used when judging your code, you can ignore other time related apis.