TypeScript：从“typeof”通用 function 获取通用 function 类型

Question

I'm pretty sure TypeScript has a way of doing this but I haven't figured it out yet.

Is there a way to extract a generic type from the typeof a generic function?

Consider this simple generic function:

function echo<T> (input: T) {
   return input;
}

I want to extract the generic type of this function. I tried:

type IEchoFn = typeof echo;

But it's unusable:

const echo2: IEchoFn = (input: string) => input;
      ^^^^^
      // Type '(input: string) => string' is not 
      // assignable to type '<T>(input: T) => T'.

I thought I'd write it as

type IEchoFn<T> = (typeof echo)<T>;

But this is invalid syntax.

How can I extract a generic type from the typeof a generic function (or its return value)?

Answer 1

Variables that has an explicit type are set in stone and can not be inferred from its value.

This is one way to solve it:

type IEchoFn<T> = (input: T) => T

const echo2: IEchoFn<string> = arg => arg