js实现返回顶部功能的解决方案

1、纯js,无动画版本

window.scrollTo(x-coord, y-coord);  
window.scrollTo(0,0);

2、纯js，带动画版本
生硬版：

var scrollToTop = window.setInterval(function() {
    var pos = window.pageYOffset;
    if ( pos > 0 ) {
        window.scrollTo( 0, pos - 20 ); // how far to scroll on each step
    } else {
        window.clearInterval( scrollToTop );
    }
}, 16); // how fast to scroll (this equals roughly 60 fps)

流畅版：

(function smoothscroll(){
    var currentScroll = document.documentElement.scrollTop || document.body.scrollTop;
    if (currentScroll > 0) {
         window.requestAnimationFrame(smoothscroll);
         window.scrollTo (0,currentScroll - (currentScroll/5));
    }
})();