JS禁用浏览器退格键实现思路及代码

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; 

 function bindBackEvent() { //防止退格键
 	$(document).keydown(function(e){
 		e = window.event || e;
	 	var code = e.keyCode || e.which;
	 	if (code == 8) {
	 		var src = e.srcElement || e.target;
	 		var tag = src.tagName;
	 		if (tag != "INPUT" && tag != "TEXTAREA") {
	 			e.returnValue = false; 
	 			return false;
	 		} else if ((tag == "INPUT" || tag == "TEXTAREA") && src.readOnly == true) {
	 			e.returnValue = false;
	 			return false; 
	 		}
	 	}
 	});
 }