You have to use a regular expressions (defined with to / delimiters or new RegExp(string)) with the methods : replace, match or test (which simply return a boolean on success).
Some caution is needed with addresses that contain special characters such as dots (which represents any characters in regular expressions and must be preceded by a backslash to regain their initial sense) and slashes (the delimiters which must too be preceded by a backslash).
Then the right syntax could be
// To replace youtube.com by youtube.com/feed/subscriptions
// To test for youtube.com/feed/subscriptions"
EDIT : With the constructor var rgx=new RegExp("myString") there is an other problem with backslashes (which mean in a string : the following characters is a control character). Then they should be doubled !