How to create a generic type function that returing value that the type is based of a returned value of a passing in function?

Question

I have a function that I can call it like this:

var a=myFunction(callback(return 33))       // a should be of type {a}
var b=myFunction(callback(return 'string')) // b should be of {type b}
var c=myFunction(callback(return [33]))     // c should be of type {number[]}

I want that a and b will be number or string or whatever based on the parameter that returns from the callback.

myFunction is calling the callback to do some calculation and returning the same type of value.

How to create the type definition for the myFunction?

Answer 1

const myFunction = <T>(callback: (...args: T[]) => T): T => {
    return callback();
};

const fnclbnum = (v: number): number => v * 2;
const fnclbstr = (v: string): string => v +'test';
const fnclbstrarr = (v: string[]): string[] => [...v, 'test'];

const a: number = myFunction<number>((val) => fnclbnum(val));
const b: string = myFunction<string>((val) => fnclbstr(val));
const c: string[] = myFunction<string[]>((val) => fnclbstrarr(val));

const d: number = myFunction<number>(() => fnclbnum(2));
const e: string = myFunction<string>(() => fnclbstr("test"));
const f: string[] = myFunction<string[]>(() => fnclbstrarr(["1", "ER"]));

console.log({ d }, { e }, { f });