I don't understand why you are doing this, but it is possible... :)
If you must insert javascript code at the begining of the tag body, you can load your javascript file like a "jsonp ajax call" and inside it call to 'prepend' jquery function.
Example:
<html>
<head>
<title>test</title>
</head>
<body>
<h1>Test</h1>
<p>Test</p>
<script type="text/javascript" src="https://code.jquery.com/ui/1.9.2/jquery-ui.min.js"></script>
<script type="text/javascript">
$( document ).ready( function() {
$.ajax({
dataType: "jsonp",
url: "myscript.js"
})
.done( function( d, response ) {
console.log( "JS LOADED" );
});
});
</script>
</body>
</html>
And in your "myscript.js" file something like:
$( "body" ).prepend( "<script>" +
"var a = 1;" +
"console.log( a );" +
"/* AND ALL THE JS CODE TO INSERT " +
"AT THE BEGINING OF TAG BODY */</script>" );
Don't forget with the semicolon (;) at the end of every js line.