Make assert method an assertion function in TS

[Andarist]

This is available since TS 3.7:
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions

It could be considered a breaking change. Note that DefinitelyTyped supports officially only the last 2 years of TS releases and that makes 4.1 the last supported version there: https://github.com/DefinitelyTyped/DefinitelyTyped#support-window

[davbrito]

I think this would be a great addition to the project. Any particular reason why it's not been merged yet?

As for the breaking change, what about adding an extra method called assertType? That way, people who are currently expecting the true return can still use assert without any problem.

[acidoxee]

I'd like to second this, the .check/.assert distinction is already native to the lib, it's just missing the proper typings. It's a bummer having either to nest the code in if statements (to benefit from the current, limited type narrowing) or having to resort to unsound casts 🙏