Javascript modification at the browser Interface

Javascript modification at the browser Interface

Is it possible to modify javascript source code on a https connection
before it gets executed by the browser. This modification doesn't have to
necessarily come from a man-in-the middle. It can also come from the
intended recipient of the script. Also, is there a particular type of
request that cannot be made to a PHP server using cURL? In other words,
what are the limitations of cURL?