Sigh. Look, get this into your head: We are not providing a JavaScript programming environment. We are providing a self-contained programming language that's totally unlike JavaScript. We also, mainly for the sake of our own development work, have an escape to JavaScript. This is useful in some libraries; it's also useful to robot developers who want to provide a way to use their robots in Snap!.
But, advance notice, we are eventually going to eliminate the JS Function block. Yes, that will break a lot of old projects, which is why we aren't doing it now. You'll have plenty of warning. The reason to eliminate it is that it's a security problem; people can (and so eventually will) write malicious projects that harm whoever runs them.
So, if all you want from Snap! is a development environment for JavaScript programming, you're in the wrong place.