页面后退键（Backspace）失效JS

1、页面引用js文件--

document.documentElement.onkeydown = function (evt) {    var b = !!evt, oEvent = evt || window.event;    if (oEvent.keyCode == 8) {//BackSpace  键        var node = b ? oEvent.target : oEvent.srcElement;        var reg = /^(input|textarea)$/i, regType = /^(text|textarea)$/i;        if (!reg.test(node.nodeName) || !regType.test(node.type) || node.readOnly || node.disabled) {            if (b) {                oEvent.stopPropagation();            }            else {                oEvent.cancelBubble = true;                oEvent.keyCode = 0;                oEvent.returnValue = false;            }        }    }}