You're confusing UnityJS with UnityScript, which is a common misconception. I wrote about the anathema that is UnityScript in another posting to this thread, and it is a completely different thing that the standard JavaScript engine that UnityJS uses.
UnityJS seamlessly supports TypeScript and all the standard JavaScript based libraries, languages, development tools, and linters, just like any browser or node.js server does, while UnityScript is nowhere near compatible enough with standard JavaScript to compile the output of the TypeScript compiler, it's not well supported by many IDEs, it's way behind the latest version of JavaScript, has non-standard half-baked extensions, and it is not capable of running any of the millions of standard JavaScript library, like UnityJS can.
So how do you like the C# and UnityScript debugging tools on the iOS, Android, and WebGL Unity3D platforms?
How does MonoDevelop compare to the standard JavaScript debuggers built into all web browsers, and their interactive consoles and data structure browsers, which support remote debugging of code running on mobile devices?
Can you set a source level breakpoint on your C# or UnityScript code that's running on an iOS device in a WebGL/WebAssembly based browser?
And how long does your typical Unity3D application take to compile and deploy, before you can see the changes you made to your code?
(I know the answers, I'm just asking rhetorically, because you know the answers too: Debugging Unity C# or UnityScript code totally sucks, especially on mobile devices and web browsers, and recompiling it is glacially slow. The MonoDevelop debugger is terrible, and it doesn't even support the il2cpp back-end, which includes iOS and WebGL, and it barely supports the deprecated Mono back-end, and crashes all the time if you can even get it to connect for a few seconds. Those are extremely painful problems that UnityJS solves.)
"UnityJS", as you call it, is rife with landmines only discovered at run-time, as one would expect from a typeless language.