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;