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using jquery library + additional .js file
the 'jquery.js' file is linked within the head tag:
<script type="text/javascript" src="path/to/jquery.js"></script>
The actual code can then be either added into the head directly via the <script type="text/javascript"> </script> tag or be written in an external .js file.
to execute the code once the document is loaded - use a READY EVENT
<script type="text/javascript">
$(document).ready(function(){
//your code here
});
</script>