Explanation of the principle of determining whether multiple elements (RADIO, CHECKBOX, etc.) are selected

var msg="";
var obj="";
var flag=false;
for (var i=0;i<;i++){
var e = [i];
if ( == 'source'){
if (==false)
flag=false;
else{
flag=true;
break;
}
}
}
if (!flag) {
msg = "Access source: Must be filled in."
obj = "source";
}