<script type="text/javascript">
// http://www.w3schools.com/jsref/jsref_obj_string.asp
// http://www.w3schools.com/jsref/jsref_slice_string.asp
// http://www.w3schools.com/jsref/jsref_eval.asp
var a1 = 'cat';
var a2 = 'dog';
var a3 = 'fish';
var a4 = 'bird';
var str = '';
for (var i = 1;i <= 4; i++){
str = str + eval('a'+i) + ',';
}
alert(str); // cat,dog,fish,bird,
str = str.slice(0, str.length-1);
alert(str); // cat,dog,fish,bird
document.write(str);
</script>