javascript username extract from email address

Hello,

I need to extract the part before @ from email address.
For example:
Field-1 (e-mail): [email protected]
Field-2 (copy from the e-mail): example

I used the following code;

<script type="text/javascript">
jQuery(document).ready(function($){
$('#field_source').change(function(){ //source field key
var sourceField = $("#field_source").val(); //source field key
$("#field_dest").val(sourceField); //destination field key
$("#field_dest").change(); //destination field key
});
});
</script>

But, it copies whole e-mail address.

So, i would like to add the following code the work it.

var name = email.substring(0, email.indexOf("@"));

How to make the complete code?

Please help.

Thanks.