What is the type of r? (TypeScript interview question)

function routes<const T extends readonly string[]>(paths: T) {
  return paths;
}
const r = routes(["/", "/about"]);

readonly ["/", "/about"]

The answer is readonly ["/", "/about"]. A const type parameter (added in TypeScript 5.0) infers the argument as if you wrote it with as const. So the literal tuple, with its exact strings, is kept.

string[] is the tempting wrong answer. You would get string[] only without the const modifier.

Generics (https://www.typescriptlang.org/docs/handbook/2/generics.html)typescriptlang.org