防抖和节流

防抖

function debounce(fn, delay, immediate = true) {
      let timer //借助闭包
      return function() {
        if(timer) clearTimeout(timer)
        if(immediate) {
          let callNow = !timer
          timer = setTimeout(() => {
            timer = null
          }, delay)
          if(callNow) fn()
        }else{
          timer = setTimeout(() => {
            fn()
            timer = null
          }, delay) // 简化写法
        }
      }
    }

节流

function throttle(fn, delay, immediate = true) {
      let previous = 0
      return function () {
        if(immediate){
          let now = Date.now()
          if(now - previous > delay){
            fn()
            previous = now
          }
        }else{
          if(!previous){
            previous = setTimeout(() => {
              fn()
              previous = 0
            }, delay)
          }
        }
      }
    }