In 20 years of software development and trying different platforms, JavaScript has been the most frustrating and the most rewarding of all of them.
I find a lot of value in using JavaScript as a cross-platform shell. I personally do a lot of work on all of the major operating systems. I primarily (a light primary, like maybe 67%, not 90%) work on Windows, but most of my users are working on macOS. And we're all writing software that eventually has to run on Android. There is almost never a problem I can't solve using a JS project of some kind. That's the rewarding part. I feel like my software goes a lot further on the JS platform than it could have ever gone on any other.
But it's frustratingly slow. A lot of that is the poor quality of a lot of the libraries that are available. I'm not really keen on writing my own HTML templating system, but I'm afraid I might have to just to get the performance targets I want (I static-gen almost everything).
And I often feel like the language is not doing enough to help me. I love writing metaprogrammable systems, but the lack of easily accessible, standardized, advanced type information in JS is a big impediment to that. Sure, I can enumerate all of the methods an object has available to it, but there is no good way to enumerate the parameters those methods take, their return type, or even that they are methods, if I ever happen to get a reference to one of them sans the object from which it came
And yes, I still do object oriented programming. But the functional story in JavaScript is not any better. With map, filter, and reduce being methods on Array, coupled with JS's goofy OO semantics where methods can become detached from their objects if they aren't explicitly bound to them, things become frustratingly verbose to get basic functional patterns to feel good.
So a WASM that let me build better dev tools, in better languages, without losing that cross-platform capability, would be a godsend. Having it run in the browser also is just icing on the cake at this point.