Retrieve the instance type using a static method in TypeScript

Question

Retrieve the instance type using a static method in TypeScript

Is it possible to name the currently class's instance type within a static method in a base class?

In other words, can you ensure type checking in the following code:

class Base {
  static addInitializer(i: (v: any /* What type would go here? */) => void) {
    // implementation is irrelevent
  }
}

class Dog extends Base {
  bark() {}
}

class Fish extends Base {
  swim() {}
}

Dog.addInitializer((dog) => {
  dog.bark();
  // @ts-expect-error
  dog.swim();
});

Fish.addInitializer((fish) => {
  fish.swim();
  // @ts-expect-error
  fish.bark();
});

Note the use of // @ts-expect-error before lines that should be a type error. This is due to the fact that v has type any rather than the desired named type.

TypeScript playground link

typescript

Answer 1

Answer №1

When you make the static method generic and constrain it by the this parameter of the static method, you allow the type of the class on which you are calling the static method to be accessible. To achieve this, simply utilize the InstanceType operator as shown below:

class Parent {
  static customize<T extends new (...args: any) => Parent>(this: T, customization: (instance: InstanceType<T>) => void) {
    // implementation details are irrelevant
  }
}

class Cat extends Parent {
  purr() {}
}

class Bird extends Parent {
  chirp() {}
}

Cat.customize((cat) => {
  cat.purr();
  // @ts-expect-error
  cat.chirp();
});

Bird.customize((bird) => {
  bird.chirp();
  // @ts-expect-error
  bird.purr();
});

Answer 2

When you make the static method generic and constrain it by the this parameter of the static method, you allow the type of the class on which you are calling the static method to be accessible. To achieve this, simply utilize the InstanceType operator as shown below:

class Parent {
  static customize<T extends new (...args: any) => Parent>(this: T, customization: (instance: InstanceType<T>) => void) {
    // implementation details are irrelevant
  }
}

class Cat extends Parent {
  purr() {}
}

class Bird extends Parent {
  chirp() {}
}

Cat.customize((cat) => {
  cat.purr();
  // @ts-expect-error
  cat.chirp();
});

Bird.customize((bird) => {
  bird.chirp();
  // @ts-expect-error
  bird.purr();
});

Retrieve the instance type using a static method in TypeScript

Answer №1

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