节流

function throttle(func, wait) {
    let timeout,self,args;
    let previous = 0;
    let later = function(){
        previous = +new Date();
        timeout = null;
        func.apply(self,args);
    }
    let throttled = function(...param) {
        self = this;
        args = param;
        let now = +new Date();
        let remaining = wait-(now-previous);
        // 如果没有剩余的时间了或者你改了系统时间
        if (remaining<=0 || remaining>wait) {
            if(timeout){
                clearTimeout(timeout)
                timeout=null;
            }
            previous = now;
            func.apply(self, args);
        }else if(!timeout){
            timeout=setTimeout(later,remaining);
        }
    };
    return throttled;
}
window.addEventListener('scroll',throttle(fn,500));