How can a TypeScript function be defined to retrieve an instance based on a specified class parameter?

Question

How can a TypeScript function be defined to retrieve an instance based on a specified class parameter?

Explanation of inheritance hierarchy:

class A {}

Classes B and C extend class A.

class B extends A {}

class C extends A {}

Using a function to create instances.

namespace D {
  function NewInstance<T extends A = A>(iclass: T): T;
}

Desired functionality: removing <T extends A> should automatically determine the return type based on the input class.

D.NewInstance(B) => B, D.NewInstance(C) => C

Current behavior: Not setting the generic results in D.NewInstance(B) => A, D.NewInstance(C) => A

typescript definition

Answer 1

Answer №1

To achieve the desired functionality, simply eliminate the default type in the declaration of the generic function.

interface X { }
interface Y extends X { }

function CreateInstance<U extends X>(instance: U): U { 
  return instance
}

const x: X = {}
const y: Y = {}

CreateInstance(x) // will result in X being returned
CreateInstance(y) // will result in Y being returned

Answer 2

To achieve the desired functionality, simply eliminate the default type in the declaration of the generic function.

interface X { }
interface Y extends X { }

function CreateInstance<U extends X>(instance: U): U { 
  return instance
}

const x: X = {}
const y: Y = {}

CreateInstance(x) // will result in X being returned
CreateInstance(y) // will result in Y being returned

Answer 3

Answer №2

Shoutout to @Karthick Vinod for the help, but in my case, the parameter for my function is a class.

Decided to tackle my own question and found that utilizing the

type Newable<T> = new (...args: any[]) => T;

is the solution.

abstract class X {
  x: number = 1;

  abstract xx(): void;
}

class Y extends X {
  y: number = 2;

  xx(): void {}
}

class Z extends X {
  z: number = 3;

  xx(): void {}
}

type Innovative<U> = new (...args: any[]) => U;

function BrandNewInstance<V extends X>(iclass: Innovative<V>): V {
  return new iclass();
}

const m = BrandNewInstance(X); // error, X cannot be instantiated

const n = BrandNewInstance(Y); // returns Y
console.log(n.x); // valid
console.log(n.y); // valid
n.xx(); // valid

const o = BrandNewInstance(Z);
console.log(o.x);
console.log(o.z);
o.xx();

Answer 4

Shoutout to @Karthick Vinod for the help, but in my case, the parameter for my function is a class.

Decided to tackle my own question and found that utilizing the

type Newable<T> = new (...args: any[]) => T;

is the solution.

abstract class X {
  x: number = 1;

  abstract xx(): void;
}

class Y extends X {
  y: number = 2;

  xx(): void {}
}

class Z extends X {
  z: number = 3;

  xx(): void {}
}

type Innovative<U> = new (...args: any[]) => U;

function BrandNewInstance<V extends X>(iclass: Innovative<V>): V {
  return new iclass();
}

const m = BrandNewInstance(X); // error, X cannot be instantiated

const n = BrandNewInstance(Y); // returns Y
console.log(n.x); // valid
console.log(n.y); // valid
n.xx(); // valid

const o = BrandNewInstance(Z);
console.log(o.x);
console.log(o.z);
o.xx();

How can a TypeScript function be defined to retrieve an instance based on a specified class parameter?

Answer №1

Answer №2

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