防抖

function debounce(func, wait, immediate) {
    let timeout,self,args,result;
    return function (...param) {
        self = this;
        args = param;
        if (timeout) clearTimeout(timeout);
        if (immediate) {
            // 如果已经执行过，不再执行
            let callNow = !timeout;
            timeout = setTimeout(function(){
                timeout = null;
            }, wait)
            if (callNow) result = func.apply(self, args)
        }
        else {
            timeout = setTimeout(function(){
                func.apply(self, args)
            }, wait);
        }
        return result;
    }
}
input.addEventListener('input',debounce(fn,500,false));