function throttle(fn, delag) {

let delagTime = delag || 200

let timer

let last

return function（）｛

let self = this

let args = arguments

let now = +new Date()

if (last && now - last < delagTime){

clearTimeout(timet)

timer = setTimeout(function(){

last = now

fn.apply(self, args)

},delagTime)

} else ｛

last = now

fn.apply(self, args)

｝

｝

}