js 禁止后退键

function doKey(e) {

var ev = e || window.event; //获取event对象
var obj = ev.target || ev.srcElement; //获取事件源
var t = obj.type || obj.getAttribute('type'); //获取事件源类型
var readonly = $(obj).attr("readonly");
try {
if (readonly === "readonly") {
return false;
}
} catch (e) {
}

if (ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea") {
return false;
}
}
//禁止后退键 作用于Firefox、Opera
document.onkeypress = doKey;
//禁止后退键  作用于IE、Chrome
document.onkeydown = doKey;
document.onkeypress = doKey;