Type signature for generic function whose argument has a different type than output

Issue

Hi I'm trying to pass in a function as an argument to another function and in doing so need to provide the type signature for that function. I thought this would do the trick:

const funct1 = (obj1: Object, funct2: <T, K>(a:T) => K, obj2: any) => {
    ///...
}

However when I call it with something like this:

const convertFromDate = (obj: Date) : number => {
  return obj.valueOf() / 1000;
};

funct1(d1, convertFromDate, Date) // error: Type 'number' is not assignable to type 'K'. 
                                  //'number' is assignable to the constraint of type 'K',
                                  // but 'K' could be instantiated with a different subtype of constraint 'unknown'.

I got the above error. How should I define the type signature of a function that takes an argument of one type and returns another?

Solution

wasn't sure what you wanted to do with obj2 in funct1. But the understanding is that you want func1 to take a func as a param, and an object that is this functions arguments. Then returns the execution of func(obj1)

type FunctionReturnType<T> = T extends (...args: any) => infer R ? R : T;
const funct1 = <T,K extends (obj: T) => any>(obj1: T, func2:K):FunctionReturnType<K> => {
    return func2(obj1);
}

const d1 = new Date()
const convertFromDate = (obj: Date) : number => {
  return obj.getTime();
};

const s = funct1<Date,typeof convertFromDate>(d1, convertFromDate) //type:number
console.log(s) //Log 166270725578...

Answered By - Frallan