The new ReturnType in TypeScript 2.8 is a really useful feature that lets you extract the return type of a particular function.
function foo(e: number): number {
return e;
}
type fooReturn = ReturnType<typeof foo>; // number
However, I'm having trouble using it in the context of generic functions.
function foo<T>(e: T): T {
return e;
}
type fooReturn = ReturnType<typeof foo>; // type fooReturn = {}
type fooReturn = ReturnType<typeof foo<number>>; // syntax error
type fooReturn = ReturnType<(typeof foo)<number>>; // syntax error
Is there a way extract the return type that a generic function would have given particular type parameters?
TypeScript compiler does not see typeof foo as generic type. I'd say it's a bug in the compiler.
However, TypeScript has callable interfaces which can be generic without any problems, so if you introduce a callable interface compatible with the signature of your function, you can implement your own equivalent of ReturnType like this:
function foo<T>(x: T): T {
return x;
}
interface Callable<R> {
(...args: any[]): R;
}
type GenericReturnType<R, X> = X extends Callable<R> ? R : never;
type N = GenericReturnType<number, typeof foo>; // number