The pros: It's RIGHT THERE. It's in your browser, complete with console and debugger. You don't need to download a compiler or interpreter or anything to play with it. You don't have to visit any particular site. All you need is Notepad or TextEdit.
It's about as close as you're going to come to this nowadays:
C64 BASIC was all there was for many people, otherwise computers were mostly not affordable and available to children. There just wasn't real choice, so we used what we had. If structured BASIC or better had been shipped that would have been the thing to use instead, without a doubt.
Today there's a choice. We can't pretend we are forced. Now there are many MUCH better things. And if we intentionally choose hardware which enforces the use of something bad to start with, when we could put all kinds of things on all kinds of hardware today - it isn't because we were forced but because we are stupid. People with experience use Javascript because they have to in order to program the browser (due to a completely artificial monopoly) and they have the ability to adapt to it. Maybe they even develop stockholm syndrome and forget all the ways it's broken and missing basic capabilities. But that doesn't mean it's a good choice among starting languages when we have a free choice.
This. When I was in middle school, I (along with many of my friends) used to program games in TI-Basic on our TI-83 calculators because it was right there and we always had our calculators at school. There were definitely better/simpler languages available, but nothing beats convenience and a low barrier to entry.
What's the problem to going to a particular site? There are loads of languages you can learn interactively online. Even if you were just learning JavaScript, you'd still have to visit specific sites for information.
It's about as close as you're going to come to this nowadays:
BASIC sucked in a lot of ways, but it was right there, inviting you to play around.