Proposal: add `toLowerCase()` and `toUpperCase()`

[jonahsnider]

String.prototype.toLowerCase() and String.prototype.toUpperCase() are typed as returning string. This doesn't make use of the relatively recently added Uppercase<T> and Lowercase<T> types in TypeScript.

I propose adding two functions to this library:

function toLowerCase<T extends string>(string: T): Lowercase<T>;

function toUpperCase<T extends string>(string: T): Uppercase<T>;

[tychenjiajun]

If both methods should be added, I propose adding endsWith, startsWtih, padEnd, padStart, repeat, trimEnd, trimStart, trim and so on. https://github.com/microsoft/TypeScript/blob/main/tests/cases/conformance/types/literal/templateLiteralTypes1.ts

[sindresorhus]

How would using this method over String.prototype.toLowerCase() improve your code? Strict types are great, but they got to have a purpose.

[jonahsnider]

Personally I don't have any use case for either of the proposed case functions. However there are surely developers who could benefit from using one of them instead of just asserting the type after using the built-in method.

[sindresorhus]

Let's keep this open for more feedback. I would like to see more realistic use-cases.