Javascript – How Prototypal Inheritance really works(vjeux)

Everywhere on the web we read that Javascript has prototypal inheritance. However Javascript only provides by default a specific case of prototypal inheritance with the new operator. Therefore, most of the explanations are really confusing to read. This article aims to clarify what is prototypal inheritance and how to really use it on Javascript.

Prototypal Inheritance Definition

When you read about Javascript prototypal inheritance, you often see a definition like this:

When accessing the properties of an object, JavaScript will traverse the prototype chain upwards until it finds a property with the requested name. Javascript Garden

Most Javascript implementations use __proto__ property to represent the next object in the prototype chain. We will see along this article what is the difference between __proto__ and prototype.

Note: __proto__ is non-standard and should not be used in your code. It is used in the article to explain how Javascript inheritance works.

The following code shows how the Javascript engine retrieves a property (for reading).

function getProperty(obj, prop) {
  if (obj.hasOwnProperty(prop))
    return obj[prop]
 
  else if (obj.__proto__ !== null)
    return getProperty(obj.__proto__, prop)
 
  else
    return undefined
}

Let's take the usual class example: a 2D Point. A Point has two coordinates x, y and a method print.

Using the definition of the prototypal inheritance written before, we will make an object Point with three properties: x, y and print. In order to create a new point, we just make a new object with __proto__ set to Point.

var Point = {
  x: 0,
  y: 0,
  print: function () { console.log(this.x, this.y); }
};
 
var p = {x: 10, y: 20, __proto__: Point};
p.print(); // 10 20

Javascript Weird Prototypal Inheritance

What is confusing is that everyone teaches Javascript prototypal inheritance with this definition but does not give this code. Instead they give something like this:

function Point(x, y) {
  this.x = x;
  this.y = y;
}
Point.prototype = {
  print: function () { console.log(this.x, this.y); }
};
 
var p = new Point(10, 20);
p.print(); // 10 20

This is completely different from the code given above. Point is now a function, we use a prototype property, the new operator. What the hell!?

How new works

Brendan Eich wanted Javascript to look like traditional Object Oriented programming languages such as Java and C++. In those, we use the new operator to make a new instance of a class. So he wrote a new operator for Javascript.

• C++ has the notion of constructor, that initializes the instance attributes. Therefore, the new operator must target a function.
• We need to put the methods of the object somewhere. Since we are working on a prototypal language, let's put it in the prototype property of the function.

The new operator takes a function F and arguments: new F(arguments...). It does three easy steps:

1. Create the instance of the class. It is an empty object with its __proto__ property set to F.prototype.
2. Initialize the instance. The function F is called with the arguments passed and this set to be the instance.
3. Return the instance

Now that we understand what the new operator does, we can implement it in Javascript.

     function New (f) {
/*1*/  var n = { '__proto__': f.prototype };
       return function () {
/*2*/    f.apply(n, arguments);
/*3*/    return n;
       };
     }

And just a small test to see that it works.

function Point(x, y) {
  this.x = x;
  this.y = y;
}
Point.prototype = {
  print: function () { console.log(this.x, this.y); }
};
 
var p1 = new Point(10, 20);
p1.print(); // 10 20
console.log(p1 instanceof Point); // true
 
var p2 = New (Point)(10, 20);
p2.print(); // 10 20
console.log(p2 instanceof Point); // true

Real Prototypal Inheritance in Javascript

The Javascript specifications only gives us the new operator to work with. However, Douglas Crockford found a way to exploit the new operator to do real Prototypal Inheritance! He wrote the Object.create function.

Object.create = function (parent) {
  function F() {}
  F.prototype = parent;
  return new F();
};

This looks really strange but what it does is really simple. It just creates a new object with its prototype set to whatever you want. It could be written as this if we allow the use of __proto__:

Object.create = function (parent) {
  return { '__proto__': parent };
};

The following code is our Point example with the use of real prototypal inheritance.

var Point = {
  x: 0,
  y: 0,
  print: function () { console.log(this.x, this.y); }
};
 
var p = Object.create(Point);
p.x = 10;
p.y = 20;
p.print(); // 10 20

Conclusion

We have seen what prototypal inheritance is and how Javascript implements only a specific way to do it.

However, the use of real prototypal inheritance (Object.create and __proto__) has some downsides:

• Not standard: __proto__ is non-standard and even deprecated. Also native Object.create and Douglas Crockford implementation are not exactly equivalent.
• Not optimized: Object.create (native or custom) has not yet been as heavily optimized as the new construction. It can be up to 10 times slower.

Some further reading:

• Douglas Crockford: Prototypal Inheritance
• MDC Documentation: __proto__
• John Resig: getPrototypeOf
• Javascript Garden: Object.prototype
• Dmitry Shoshnikov: OOP: ECMAScript Implementation
• Angus Croll: Understanding Javascript prototypes
• Yehuda Katz: Understanding JavaScript Function Invocation and “this”

Bonus

If you can understand with this picture (from the ECMAScript standard) how Prototypal Inheritance works, you get a free cookie!