Most non-copyleft free licences have requirements similar to these from the 3-clause BSD licence:
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
Most JS used on the internet is minified before publication, and this process normally removes all comments, including copyright notices, along with other "extraneous" material. Minified JS isn't exactly binary code, but it's not exactly source code within the traditional meanings of free software, either; GPLv3 defines source code as "the preferred form of the work for making modifications to it", and minified JS is hardly that. In any case, whichever form you feel it is, the copyright notice and a short text passage must go along with it.
As the leading answer on SE.SE points out, most minifying software has an option to leave certain extraneous text in the input untouched. You should find out how you instruct your minifier of choice to do this, and ensure that whatever you are required to pass on untouched by the licence is left untouched.
If you unwilling or unable do that, you should be aware that you are violating copyright. It is highly unlikely there will be any comeback for that, but if there should be, neither "I like my minifier and I couldn't work out how to make it preserve the copyright notices" nor "My boss told me to cut out the extra 1500 bytes" will be an acceptable excuse.