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backtop.js

var backtop = document.getElementById('backtop');//need a button id is 'backtop'
backtop.onclick = function () {
    backTimer = setInterval(_back, 30);
};
var backTimer;
function _back () {
    var _body = document.documentElement;
    if (!_body.scrollTop) {
        _body = document.body;
    }
    _body.scrollTop -= 50;
    if (_body.scrollTop <= 0) {
        _body.scrollTop = 0;
        clearTimeout(backTimer);
    }
}