节流防抖

1. 节流–throttle,隔一段时间触发一次
   用定时器实现节流，由于定时器的延迟性，最后可能还执行一次

 function throttle1(fn, delay) {
        let timer = null
        return function () {
          let args = arguments
          let context = this
          if (!timer) {
            timer = setTimeout(() => {
              fn.apply(context, args)
              timer = null
            }, delay)
          }
        }
      }

时间戳节流，高频操作，最后一次时间间隔小，函数不在触发

    function throttle2(fn, delay) {
        let timer = null
        let last = Date.now()
        return function () {
          let args = arguments
          let context = this
          let now = Date.now()
          if (now - last >= delay) {
            fn.apply(context, args)
            last = now
          }
        }
      }

2. 防抖debounce，时间间隔内触发，定时器不断清除，只执行最后一次

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

3. 由于防抖只触发最后一次，中间没有响应，看起来像是页面卡死了，所以用节流优化下，打造一个有底线的debounce：delay时间内，还是不断创建新的定时器清除旧定时器，超出delay时间后，必须给一个响应

  function betterDebounce(fn, delay) {
        let timer = null,
          last = 0
        return function () {
          let args = arguments
          let context = this
          let now = Date.now()
          if (now - last < delay) {
            clearTimeout(timer)
            timer = setTimeout(() => {
              fn.apply(context, args)
              last = now
            }, delay)
          } else {
            fn.apply(context, args)
            last = now
          }
        }
      }

	function fn() {
        console.log("hahahahah")
      }
      window.addEventListener("scroll", betterThrottle(fn, 1000))