``` js
function throttle(fn, wait) {
let lastTime = 0
let timer = null
let nowTime = new Date().getTime();
let params = null
return function (...args) {
params = args;
if (nowTime - lastTime > wait) {
fn.apply(this, args)
lastTime = nowTime
} else if (timer === null) {
timer = setTimeout(() => {
timer = null
fn.apply(this, params)
}, wait)
}
}
}
```

_.countBy