Whereas, in PHP, it’s possible to:
<?PHP if($foo) { ?>
<strong>only appears if $foo is true.</strong>
<?PHP } else { ?>
<em>only appears if $foo is false.</em>
<?PHP } ?>
Can one, in Javascript, open a pair of curlies in one script tag that get closed in another?
The short answer is: No.
The long answer is javascript scripts are compiled and executed at the closing of the script tag (not when the document has finished loading) which means that any incomplete code is and has always been treated as a syntax error.