1. By far, has the largest ecosystem.
2. If you use a linter, a typechecker (TS or flow), and transpile your code (TS or babel or both), JavaScript is actually a pretty nice language to work in.
Sure it's a chore to set up all the initial tooling, and learn the ins and outs. However, once you're past that, it's usually not worth it to switch.
2. It's still far from the state of the art in PL design and missing:
* Nice static typing with type inference
* Nice guarantees about null access
* Nice guarantees about interop between client and server
* ...
>However, once you're past that, it's usually not worth it to switch.
That is somewhat circular and can be said of virtually anything. That is, for every minute you invest in a tech, the degree to which it is worthwhile to switch to another tech diminishes. I'm speaking primarily about tech like frameworks that is aimed at making us more productive.
The exception might be a truly revolutionary new tech that sharply boosts productivity, but also has a very low learning curve. However, that scenario seems particularly unicorn-ish where JS frameworks are concerned.