<script>
String.prototype.replaceAll = function (AFindText,ARepText){
                raRegExp = new RegExp(AFindText,"g");
                return this.replace(raRegExp,ARepText);
}
alert("axxxbxxxxcxxxxx".replaceAll("x", "_"))
</script>

------解决方案--------------------

一楼写的是很好的，请看我下面的实例代码：
<script language="javascript" type="text/javascript">
  String.prototype.replaceAll = function (AFindText,ARepText){
               raRegExp = new RegExp(AFindText,"g");
               return this.replace(raRegExp,ARepText);
				}
	var str = "123,456,789,4";	
	alert(str.replaceAll(",",""));
</script>
运行结果：1234567894


------解决方案--------------------
写一个方法:

function deleDot(){
           var str = "1,2,3";
           var newStr = "";
           for(var i=0; i<str.length; i++){
               if(str.charAt(i) != ","){
                  newStr += str.charAt(i);
               }
           }
           alert(newStr);
       }