Timer

createTimer.js

const createInterval = () => {
  let interval = null
  const start = (cb, delay) => !interval && (interval = setInterval(cb, delay))
  const stop = () => {
    clearInterval(interval)
    interval = null
  }
  return { start, stop }
}

const createTimer = (initial, delay = 1000) => {
  const interval = createInterval()
  let duration = initial
  let lastPlay = null

  const play = (cb) => {
    interval.start(() => {
      cb((duration += new Date().getTime() / 1000 - lastPlay))
      lastPlay = new Date().getTime() / 1000
    }, delay)

    lastPlay = new Date().getTime() / 1000
  }
  const pause = () => interval.stop()
  return { play, pause }
}