How to extend function constructor with class

November 5, 2023

Is there any easy way to inherit function constructor from class?

For example I have this code:

class Parent {
  constructor() {
    this.parentField = 1;
  }
}

function Child() {
  // Parent.call(this); // TypeError: class constructors must be invoked with 'new'
  
  this.childField = 2;
}

Object.setPrototypeOf(Child, Parent);
Object.setPrototypeOf(Child.prototype, Parent.prototype);

const child = new Child();

console.log(child);

As you can see the problem is when I try to call constructor of class

The easiest way which I came up is to move code from constructor into class.prototype.initialize:

class Parent {
  constructor() {
    this.initialize();
  }
  
  initialize() {
    this.parentField = 1;
  }
}

function Child() {
  Parent.prototype.initialize.call(this);
  
  this.childField = 2;
}

Object.setPrototypeOf(Child, Parent);
Object.setPrototypeOf(Child.prototype, Parent.prototype);

const child = new Child();

console.log(child);

What if the class is written in some library or for some reason I don’t have access to this class?

The easiest way which I came up is to "recreate" the constructor of class:

class Parent {
  constructor() {
    this.parentField = 1;
  }
}

function ParentConstructor(...args) {
  const parent = new Parent(...args);

  for (const key of Object.getOwnPropertyNames(parent)) {
    this[key] = parent[key];
  }
  
  // or
  // Object.setPrototypeOf(this, parent);
  // but in this case I will not see the public keys of parent
}

function Child() {
  ParentConstructor.call(this);

  this.childField = 2;
}

Object.setPrototypeOf(Child, Parent);
Object.setPrototypeOf(Child.prototype, Parent.prototype);

const child = new Child();

console.log(child);

I want to know are there better ways to inherit function constructor from class for both situations:

I have access to class
I haven’t access to class

>Solution :

Inheriting a class constructor in a function can be achieved by calling the class constructor within the function and manually setting the prototype chain.

class Parent {
  constructor() {
    this.parentField = 1;
  }
}

function Child() {
  let instance = Reflect.construct(Parent, arguments, new.target || Child);
  instance.childField = 2;
  return instance;
}

Object.setPrototypeOf(Child.prototype, Parent.prototype);

const child = new Child();

console.log(child);