获取多个checkbox的值，并用逗号隔开

var S1=""; //初始化  
var yp = document.getElementsByName("yp");   
for(var i = 0 ; i < yp.length ; i++){  
    if(yp[i].checked){      
        S1 += yp[i].value+",";   
    }   
}   
S1 = S1.substring(0,S1.length-1);