防抖和节流

答：防抖：用户触发事件过于频繁，只要最后一次事件的操作

function debounce（fn, t1）{
let t = null
return function () {
if (t) clearTimeOut t
t = setTimeOut( (this) => {
fn.call(this)
},t1)
}
}

节流：指一定时间内只执行一次
function throttle （fn, t1）{
let previous = 0
return function () {
let now = new.Date().getTime()
if (now - previous > t1) {
fn.call(this)
previous = now
}
flag = false
}
}