String case change methods should return intrinsic string manipulation types

lib Update Request

Configuration Check

My compilation target is ES2018 and my lib is ["ES2016", "dom"].

Missing / Incorrect Imprecise Definitions

At least toUpperCase and toLowerCase methods on String, maybe/probably also the Locale versions?

Sample Code

type LittleT = "t" | "tt";
type BigT = Uppercase<LittleT>;
declare const smol: LittleT;
const big: BigT = smol.toUpperCase(); // err, string is not assignable to "T" | "TT

Documentation Link

https://tc39.es/ecma262/#sec-string.prototype.tolowercase

Note

I’m not sure about the Locale versions of these methods because I don’t know what algorithm the “intrinsic” Uppercase<T> / Lowercase<T> helper types are required to follow. (Do we need separate LocaleUppercase / LocaleLowercase helpers?)

Also worth mentioning: I think what I’m looking for is a return type of e.g. Uppercase<this>, which shouldn’t have an impact on non-literal string types because Uppercase<string> is just string.

[ajafff]

Just use a generic this type on the method signatures:

interface String {
    toUpperCase<T extends string>(this: T): Uppercase<T>;
}

[mycarrysun]

This issue is “awaiting more feedback”. This means we’d like to hear from more people who would be helped by this feature, understand their use cases, and possibly contrast with other proposals that might solve the problem in a simpler way (or solve many other problems at once).

If this feature looks like a good fit for you, please use the 👍 reaction on the original post. Comments outlining different scenarios that would be benefited from the feature are also welcomed.

Wanted to comment as you mentioned you wanted to hear if others find this useful. I was searching for a workaround for this exact scenario so yes it helps me a lot. We are eslint banning type assertions in our code base so would rather not have to do type assertions and augmenting built in types is not a great solution. Plus this is the expected behavior since this is how the function actually works.