原生javascript禁用和屏蔽鼠标右键

(function(){
    var doc=document,
        ua = navigator.userAgent.toLowerCase(),
        check = function(r){return r.test(ua);},
        isOpera = check(/opera/),
        isChrome = check(/chrome/),
        isWebKit = check(/webkit/),
        isSafari = !isChrome && check(/safari/),
        isSafari2 = isSafari && check(/applewebkit/4/),
        isSafari3 = isSafari && check(/version/3/),
        isSafari4 = isSafari && check(/version/4/),
        isIE = !isOpera && check(/msie/),
        isIE7 = isIE && check(/msie 7/),
        isIE8 = isIE && check(/msie 8/),
        isIE9 = isIE && check(/msie 9/),
        isIE10 = isIE && check(/msie 10/),
        isIE11 = isIE && check(/msie 11/),
        isIE6 = isIE && !isIE7 && !isIE8 && !isIE9 && !isIE10 && !isIE11,
        isGecko = !isWebKit && check(/gecko/),
        isGecko2 = isGecko && check(/rv:1.8/),
        isGecko3 = isGecko && check(/rv:1.9/);
    function preventKey(e) {
        var ev = e || window.event,//获取event对象  
        obj = ev.target || ev.srcElement,//获取事件源  
        t = obj.type || obj.getAttribute('type'),readonly = obj.readOnly||obj.getAttribute('readonly'), code = ev.keyCode||ev.which||ev.charCode,charcode = String.fromCharCode(code).toLowerCase();//获取事件源控件类型,控件只读属性，键盘值
        if ( (code == 8 && t != "password" && t != "text" && t != "textarea")||(readonly&&(t == "password" || t == "text" || t == "textarea"))) {//除不只读输入框外禁止后退键
            return false;
        }
        if (((isOpera || isGecko)?ev.which==0:true)&&(code == 116 || code == 122|| code == 123 || (ev.shiftKey && code == 121))||(((isOpera || isGecko)?ev.which!=0:true)&&ev.ctrlKey && (charcode == 'a' || charcode == 's'))) {//屏蔽 F5,F11,F12,shift+F10,ctrl+a,ctrl+s
            if (isIE)
                ev.keyCode = 0;
            ev.returnValue = false;
            return false;
        }
    };
    if (isOpera || isGecko)
        doc.onkeypress = preventKey;
    else if (isIE || isChrome || isSafari)
        doc.onkeydown = preventKey;
    if(isIE)document.onselectstart=function(){return false;};
    doc.oncontextmenu = function(){
        if(window.event){
            window.event.cancelBubble = true;
            window.event.returnValue=false;
        }
        return false;
    };
    try{window.history.forward(1);}catch(e){}
})();