Javascript create objects with both new and without new + inheritance

Question

I am creating a library in javascript that create javascript objects.

How can I write the interface of the library so that their users can create such objects both WITH and WITHOUT new? (I saw a lot of answers that propose constructors that auto-call itselfs with new if they were not invoked with new in first place but no the other way around).
Can we use new with Object.create? For example: let dog = new Object.create(animal);
How to provide inheritance

To illustrate with code, how do you write the functions Animal and Dog below so that the following expressions are valid:

let animal = new Animal(); // valid
let animal = Animal(); // valid also, we should return the same object
let dog = new Dog(); // valid, dog inherits/shares functions and properties from Animal.
let dog = Dog(); // valid also, same case as in previous call.

Thank you so much.

If a constructor returns an object, new will have no effect. Perhaps look into that. Example. — ibrahim mahrir
– ibrahim mahrir, Commented Apr 21, 2017 at 14:51
So can I for instance directly return Object.Create? and it will work (including inheritance) with and without new? — Dconversor
– Dconversor, Commented Apr 21, 2017 at 17:05

BabarConnect · Accepted Answer · 2017-04-21 15:36:32Z

3

I would do :

function Animal(name) {
  if(!(this instanceof Animal)) {
    return new Animal(name);
  }

  this.name = name;
}

Animal.prototype.walk = function() { console.log(this.name, 'is walking...'); };

function Dog(name) {
  if(!(this instanceof Dog)) {
    return new Dog(name);
  }

  this.name = name;
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

var animal = Animal('John');
var other_animal = new Animal('Bob');

var dog = Dog('Blue');
var other_dog = new Dog('Brutus');

animal.walk(); // John is walking...
other_animal.walk(); // Bob is walking...

dog.walk(); // Blue is walking...
other_dog.walk(); // Brutus is walking...

console.log(dog instanceof Animal); // true
console.log(dog instanceof Dog); // true

edited Apr 21, 2017 at 15:36

answered Apr 21, 2017 at 14:57

BabarConnect

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9 Comments

ibrahim mahrir Over a year ago

You have to set the constructor of Dog to Dog after Dog.prototype = Animal.prototype. And don't forget to put in some semicolons.

Steven Liekens Over a year ago

I think you set Animal.prototype.constructor to Dog because you didn't clone the prototype.

Steven Liekens Over a year ago

I'm wondering if there's a way to set name on Animal.prototype so that Dog inherits it .

BabarConnect Over a year ago

@StevenLiekens I think Animal.call(this, name); in the Dog constructor (replacing this.name = name;) will do it

Steven Liekens Over a year ago

That still ends up assigning to this.name which is different from Dog.prototype.name. Maybe that's the correct behavior though.

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