js 手写篇-手写节流

节流是在一定时间段内的大量操作集合成一次操作，在一个时间段有大量重复操作，那么只会执行一次，节流可以用到scroll监听上

let box1 = document.querySelector('.box2') 
let box2 = document.querySelector('.box1');
function play() {
  console.log(Math.floor(Math.random()*100));
}
function throttle(fn,delay){
  let prev = Date.now();
  return function(){
	let context = this,args = arguments;
	let now = Date.now();
	if(now - prev > delay(){
	  fn.apply(context,args);
	  prev = Date.now();
	}
  }
}
box2.addEventListener('scroll',throttle(play,1500))