节流、防抖

节流 ：在单位时间内，只触发一次。eg.scroll

function throttle(fn,delay=500){
  let flag = true
  return function(...args){
    let context = this
    if(!flag) return 
    flag = false
    setTimeout(()=>{
      fn.apply(context,args)
      flag = true
    },delay)
  }
}

防抖：事件触发n秒后再执行回调，n秒内触发，重新计时。eg.resize|change

function debounce(fn,delay=500){
  let timer = null
  return function(...args){
    let context = this
    timer && clearTimeout(timer)
    timer = setTimeout(()=>{
      fn.apply(context,args)
    },delay)
  }
}