Question

<script id="s1" src="foo.js"></script>
<script>
    alert('foo.js contains' + _source_code_of('s1'))
</script>

Can _source_code_of be implemented?

Answer 1

No, this would allow to retrieve the contents of any URL, which would break some security policies. (This would be an equivalent of an ajax get request without same-domain checks.)

However, since foo.js is on the same domain than the page you can fetch it with an ajax request. Example with jQuery:

$.get('foo.js', function(source_code) {
    alert('foo.js contains ' + source_code);
});