点击回到顶部

var oBack = document.getElementById("back");  //回到顶部的按钮
window.onscroll = function(){
    if (getScrollTop() >= 500) {
        oBack.style.display = "block";
    }else{
        oBack.style.display = "none";
    }
}
//点击回到顶部
var timer;
var flags = true;
oBack.onclick = function(){
	
    if (flags == true) {
        flags = false;
    }else{
        return;
    }
    
    var t = 0;
    var b = getScrollTop();
    var c = -b;
    var d = 50;
    timer = setInterval(function(){
        t++;
        var temp = Tween.Cubic.easeIn(t,b,c,d);  //依赖于T尾恩JS，地址http://www.createjs.cc/tweenjs/
        if (t==d) {
            clearInterval(timer);
            flags = true;
        }
        document.body.scrollTop = document.documentElement.scrollTop = temp;
    },15);
}

//获取滚动条高度
function getScrollTop(){
    return document.body.scrollTop || document.documentElement.scrollTop;
}