var astr = "a,b,c,d";
var bstr = "a,c,f,e,w"; 

var s = "," + astr + ","
var a  = bstr.split(",");
for(var i=0;i<a.length;i++){
   if(s.indexOf(","+ a[i] +",")==-1){
      s += a[i] + ",";
   }    
}
s = s.substr(1,s.length-2);
alert(s)

------解决方案--------------------
[Quote=引用:] 
 var astr =  "a,b,c,d, " 
 var bstr =  "a,c,f,e,w "; 
 我想要的结果是  "a,b,c,d,f,e,w " 
 请高手帮忙 
 [/Quote] 
 var astr =  "a,b,c,d, " 
 var bstr =  "a,c,f,e,w "; 
 var arr = bstr.split( ", "); 
 for(var i=0;i <arr.length;i+=1){ 
  if(!astr.contains(arr[i])) 
    astr += ", "+arr[i]; 
 }

------解决方案--------------------
JScript code
 
  <script type="text/javascript"> 
  <!-- 
 	var astr = "abcd"; 
 	var bstr = "a,c,f,e,w";  
 	for(var i = 0, result = bstr.replace(/,/g,''); i  < result.length; i++) 
 		(astr.indexOf(result.charAt(i)) == -1) && (astr+=result.charAt(i)); 
 	alert(astr.split('')) 
  
 //--> 
  </script> 
 


------解决方案--------------------
JScript code

var astr = "a,b,c,d," 
var bstr = "a,c,f,e,w"; 
var b= bstr .split(",");
for(var   i=0;i <b.length;i+=1){ 
  if(! astr.contains(b[i])) 
      astr   += ", "+b[i];