Signature for a generic function that takes an input of one type and returns an output of a different type

Question

Signature for a generic function that takes an input of one type and returns an output of a different type

Hello, I am currently facing an issue where I am trying to pass a function as an argument to another function and need to provide the type signature for that function. Initially, I attempted to solve this problem using the following code snippet:

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

However, when I try to call it with a function 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'.

The above error message was displayed. How can I properly define the type signature of a function that takes an argument of one type and returns another?

typescript typescript-typings typescript-generics

Answer 1

Answer №1

In the context of funct1, there seemed to be some ambiguity regarding the treatment of obj2. The basic idea, however, is for func1 to accept a function as a parameter along with an object that serves as arguments for this function. Then, it returns the result of executing func(obj1).

type ReturnTypeFromFunction<T> = T extends (...args: any) => infer R ? R : T;
const funct1 = <T,K extends (obj: T) => any>(obj1: T, func2: K): ReturnTypeFromFunction<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) // Output: 176585032147...

Answer 2

In the context of funct1, there seemed to be some ambiguity regarding the treatment of obj2. The basic idea, however, is for func1 to accept a function as a parameter along with an object that serves as arguments for this function. Then, it returns the result of executing func(obj1).

type ReturnTypeFromFunction<T> = T extends (...args: any) => infer R ? R : T;
const funct1 = <T,K extends (obj: T) => any>(obj1: T, func2: K): ReturnTypeFromFunction<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) // Output: 176585032147...

Signature for a generic function that takes an input of one type and returns an output of a different type

Answer №1

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