If your client gets pissed off by your licensing policy, that doesn't mean
free software is some kind of bullshit that is not reality-based. It just
means your client has false preferences. In theory, it would not be bad for
anyone if the JavaScript you write for a webpage is licensed under the GNU
GPL.
If you write a new JavaScript function for the client, he gets what he wants
and pays you, once, for doing that job. That's all that matters: you getting
the payment for the manhours, the client getting the functionality he
desires.
So what if someone can reuse it? But that's as far theory goes of course.
It's your business and your code and you decide how to license it. If I were
you, I'd ask the client if he is ok with the GNU GPL. Point out the
possibilities he will have while getting free software: sharing it with
whomever, for example.
