防抖&节流

节流：按单位时间段(delay)去稀释事件

function throttle(fn, delay) {
  let firstTime = true;
  let timer = null;
  return function() {
    if (firstTime) {
      fn.apply(this, arguments);
      firstTime = false;
      return
    }
    if (timer) {
      return
    }
    timer = setTimeout(() => {
      fn.apply(this, arguments)
      timer = null
    }, delay)
  }
}

防抖：在单位时间间隔，只触发最后一次

function debounce(fn, delay) {
  let timer = null;
  return function() {
    if (timer) {
      clearTimeout(timer)
    }
    timer = setTimeout(() => {
      fn.apply(this, arguments)
    }, delay)
  }
}