String methods properly infering types

[KajSzy]

Methods such as toLowerCase and toUpperCase do not properly infer union type that are invoked on.

Typescript Playground Link

There is solution for that, but due to those methods being very ground level it may break things in legacy code.
Thus official typescript is not so keen to change those types.

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

[gustavoguichard]

Folks, if you want to go nuts with strongly-typed string functions, check out string-ts library =)