To give you an example from a popular open source ts-heavy project:
https://github.com/elysiajs/elysia/blob/94abb3c95e53e2a77078...
The `return this as any` there, which effectively casts it to the same type this had, but with the added get route is perfectly safe, it works, and will never be a problem by itself.
Tbh I'm not paid by this company enough to spend time breaking down the correct solution but it would involve validating and discriminating the result of `this.add`.
This code is NOT type-safe and will be harder to maintain. Someone will make a change in the future that changes the possible results of `this.add` and TypeScript will not be able to warn you about the consequences of that change.
If you were going to rely on that anyway, why not just use JavaScript as is and avoid the boilerplate from typescript
It's not like using js at all, not that I think there's anything wrong with it, if that's your jam.
I ever used Typescript because I hoped it would catch some bugs that I didn't (and it did). I never expect it to be 100%. I can't even imagine 100% of what.
Typescript is "good enough" at it's job. That's a great reason to use it.