JS禁用浏览器退格键

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 function forbidBackSpace(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     
            //获取作为判断条件的事件类型     
            var vReadOnly = obj.readOnly;    
            var vDisabled = obj.disabled;    
            //处理undefined值情况     
           vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;    
           vDisabled = (vDisabled == undefined) ? true : vDisabled;    
           //当敲Backspace键时,事件源类型为密码或单行、多行文本的,     
           //并且readOnly属性为true或disabled属性为true的,则退格键失效     
           var flag1 = ev.keyCode == 8 && (t == "password" || t == "text" || t == "textarea") && (vReadOnly == true || vDisabled == true);    
           //当敲Backspace键时,事件源类型非密码或单行、多行文本的,则退格键失效     
            var flag2 = ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea";    
            //判断     
            if (flag2 || flag1) return false;    
        }    
        //禁止后退键 作用于Firefox、Opera    
       // document.onkeypress = forbidBackSpace;    
        //禁止后退键  作用于IE、Chrome    
        document.onkeydown = forbidBackSpace;