The JavaScript file is just a plain text file. You could simply
read the file line by line.
Finding comments could simply be done with String.indexOf(), or with regular expressions. You'd probably need to be a little bit more sophisticated than just search for /* or //, because those characters could also appear inside a JavaScript
string literal, in which case they don't represent the beginning of a comment. Also, I don't know how JavaScript deals with nested comments:
/* Hello world /* Nested comment */ Some more text */
Does the first or the second */ indicate the end of the comment? You'd have to look that up somewhere in a JavaScript specification.