JavaScript prototype-based inheritance visualized

This post is actual only for:

• ECMA-262 5th edition called ECMAScript 5 (ES5)
• ECMA-262 6th edition called ECMAScript 2015 (ES6)

This is a beginner-level post to clarify and visualize JavaScript prototype-based inheritance.

Motivation

A lot of incomplete and even wrong info can be found on Internet about JavaScript prototypal inheritance. I will just try to explain it again with help of diagrams.

Understanding JavaScript inheritance mechanics is important, even if you don't plan to use JavaScript OOP patterns, since many of language built-in functionality based on inheritance.

I am not advocating of using OOP patterns and "classical"-like inheritance in JavaScript at all. I personally prefer using "factory" and "mixin" instead of a "constructor" pattern. But this post is not about patterns, it is only about JavaScript prototype-based inheritance mechanics visualization.

Diagrams notation:

• Blocks are JavaScript objects
• Title of the block denotes an accessor to that object
• All other sections in a block are properties of this object
• Arrows are references, with meaning, that given property holds reference to a pointed object. Source of arrow is important, it identifies property, but end is not, it is always points to another object
• Prototype chain, which is used by JavaScript inheritance system is colored in red
• Built-in porperties, are not listed and shortened to <built-ins>

TL;DR

This post is all about these diagrams:

If you understand them completely - you have done, if not - below is a more detailed explanation.

Some JavaScript basics

Basic part

• In JavaScript there are functions and objects
• There is no classes and no constructors in a language. ES6 class is only a syntactical sugar
• There are no methods or members in an object, there are only properties. Exception: get() and set() special syntax.
• Object property can hold a value or reference to another object or function
• Functions are also objects, but of special type
• Any function can be invoked as a constructor, but this doesn't mean it should be invoked as a constructor
• Functions, which are intended to be used as constructors, just called constructor functions. Tey have to be invoked with a new keyword to construct a new object
• By convention, constructor functions are named with PascalCase, all other functions are named with camelCase
• Function in JavaScript is a main force. It can be used as:
  • a function
  • an instance method
  • a static method
  • an object, because it is an object
  • a constructor
  • a name space
  • a closure to capture context
  • ...

Advanced part

Functions

• Every function declaration immediately creates TWO OBJECTS:
  • the function object itself
  • the prototype object, owned by this function
  • That happens before any code execution even begins, just after code parsing
• function object can be accessed just using function name without parenthesis, for example: myFunction
• prototype object can be accessed using prototype property of function object, for example: myFunction.prototype
• prototype object is used by JavaScript, when function is invoked as a constructor (with new keyword) to initialize newly constructed object __proto__ property
• prototype object of constructor function is reminiscent of what is usually stored in class definition, in classical OOP languages like Java and C++
• constructor function and its prototype object are always come togehter
• prototype object does not used at all, if function is not intended to be used as a constructor

Objects

• Every object has a built-in __proto__ property
• __proto__ property correspond to internal, hidden [[Prototype]] property of the object
• function object and its prototype object, both, also have __proto__ property
• __proto__ property as an accessor, standardized only in ES6. In ES5, existance of __proto__ property depends on implementation. In ES5 standard way to access value of [[Prototype]] property is Object.getPrototypeOf() method
• In ES6 __proto__ property can be set, it just holds reference to another object. In ES6 there is also a Object.setPrototypeOf() method
• It is possible to create object with __proto__ property set to null using var obj = Object.create(null)
• Object, which is referenced by __proto__ property of a given object, is called its parent. That parent object can also have __proto__ property to its own parent, thus forming prototype chain
• prototype chain of objects or prototypal inheritance chain is a way, how inheritance is implemented in JavaScript
• When JavaScript runtime looks for a property, with a given name, on an object, it first examines object itself, and then all objects down its prototype chain

Built-in constructor functions

This is a list of most popular JavaScript built-in constructors. They are constructors, not just functions, objects or namespaces - this is important!

• Array
• Boolean
• Date
• Error, and its derrivatives
• Function
• Math
• Number
• Object
• RegExp
• String

More reading: [MDN Standard built-in objects] (https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects)

Most confusing, of course, are Function and Object. Technically, they both are functions, constructor functions.

"Function" and "Object" terms mess

Close your eyes and take it as given.

JavaScript authors have named a function, with the name Object(). Then, they have made another function in the system, with the name Function(). And then, they have made technically every function in the system to be an object.

Of course, this is confusing. But, only for the first time. Then, you get used.

Speaker always need to be precisely clear, what he is talking about.

Meanings of term "object":

• Built-in Object constructor
• Specific JavaScript object, referenced by some access path
• A constructor with its prototype
  • The problem here in the fact, that we have no classes, and JavaScript developers often call it object but with meaning type. Better not to use terms class, type, object, object type, but use only terms constructor or constructor function
• JSON object
  • JSON stands for "JavaScript Object Notation"".
  • Typical misuse and misunderstanding is, that JSON is not an object, it is always a string, which will become an object in the memory only after parsing
• POJO, which stands for "Plain Old Javascript Object", or just "simple object"
  • This is an object without any custom prototype chain, or any added "methods", just a container for data
  • Its __proto__ property refers directly to Object.prototype, or equal to null
  • Can be considered as a Hash

Meanings of term "function":

• Built-in Function constructor
• Specific named JavaScript function, referenced by name created with function declaration
• Specific anonymous JavaScript function, referenced by some access path

Function and Object constructors relation

Relation between Function and Object constructors is very important. It plays major role in JavaScript inheritance.

To summarize:

• Every function in JS is an object, more exactly - two objects: function itself and its prototype
• There are two distinct built-in constructor functions Function and Object related to each-other
• Function.prototype object itself inherits from Object.prototype
• Every function in the system inherits from Function.prototype
• Every object in the system inherits from Object.prototype

The prototypal inheritance chain is drawn in red.

As you may see Function and Object are both functions, thus, they both have prototype property, which holds reference to respective prototype object.

Function and Object are both functions, thus their __proto__ property, refers to Function.prototype, which itself has __proto__ property referencing to Object.prototype.

Both prototype and __proto__ properties of a Function refer to the same Function.prototype object, which is a unique situation, valid only for built-in Function constructor.

"Prototype" term mess

When one says word "prototype", it immediately starts a real mess in heads of audience. Speaker always need to be precisely clear, what he is talking about.

Meanings of term "prototype":

• A "prototype" of a given object
  • Its parent
  • Accessible with someObject.__proto__ property, not prototype property
  • The most confusing part is:
    • Parent of a someObject object, referenced by someObject.__proto__ property, is commonly called its prototype
• A prototype object of a given function, especially a constructor function
  • Accessible with SomeConstructor.prototype property
  • That should be called constructor prototype
• A built-in Function.prototype object
• A built-in Object.prototype object
• A prototype of any other built-in constructor, for example String.prototype

To summarize:

• Only a function may have prototype property
• Any function has prototype property
• prototype property of a function holds reference to an auxiliary object, which is used only, when the function is invoked as a constructor, with new keyword, and completely ignored for all other regular functions
• Any object has prototype chain
• Prototype chain is built using __proto__ property, not prototype property
• Functions are also objects, and thus have __proto__ property, referencing directly to Function.prototype built-in object. Their prototype chain is always someFunction -> Function.prototype -> Object.prototype
• All prototype chains ends with Object.prototype
• Object.prototype.__proto__ holds null. This is real end of prototype chain

Function in JavaScript

Having simple function declaration, we get following inheritance.

function foo(){}

What we can see:

• function declaration creates two objects: foo itself and foo.prototype, even if foo does not going to be used as a constructor
• foo inherits directly from Function.prototype
• foo.prototype inherits from Object.prototype
• this inheritance valid for any, even anonymous or arrow function.

What we don't see is that foo itself has internal [[Code]] property, which cannot be accessed but is used when we invoke it with foo().

When you use foo.someMethod(), all built-in methods come from Function.prototype and down the chain from Object.prototype. But foo.someMethod() never comes from foo.prototype. See Static methods.

foo.prototype typically does not used at all, if function is not a constructor, and vice versa, is used in the case of a constructor function.

foo.prototype can be set to any other object reference or primitive value. Setting it to a new object or define new properties on it is a common pattern to define a class.

ES6 arrow function do not have prototype object created by default. It cannot be used as a constructor because it has lexical this and cannot initialize new instance properties.

Creating simple objects with inheritance

Simple objects in JavaScript created with object literal syntax or with Object.create function.

// Simple object literal
var foo = {
  a: 10
};

// foo object will be used as prototype for bar
var bar = Object.create(foo, {
    b: {
      value: 20
    }
  });

console.log(bar.__proto__ === foo); //true
console.log(bar.a); // 10 - Inherited property
console.log(bar.b); //20 - Own porperty
console.log(foo instanceof Object); // true, foo inherits from Object
console.log(bar instanceof Object); // true, bar inherits from Object

Important moment here is that in case of changing bar.a value, JavaScript automatically creates bar.a own property with new value, to prevent prototype pollution.

Even if prototype chain of foo and bar looks very simplistic, we can note, that both have an inherited constructor property, which points to the Object constructor, which itself inherits from Function.prototype. More of that, there are a lot of built-in methods in Object.prototype itself, not displayed for clarity. They all are accessible on foo and bar.

Creating an object with constructor function

Now, let's declare a simple constructor function and create an object instance using it.

function Bar() {
  // "this" will point to newly created object
  this.a = 10;
}

Bar.prototype.readA = function () {
  // "this" will point to the object, in context of which, method is invoked
  return this.a;
}

Bar.prototype.writeA = function (a) {
  this.a = a;
}

// Constructor function invocation requires "new" keyword
var bar = new Bar();

console.log(bar.constructor === Bar); // true, "constructor" - inherited property
console.log(bar instanceof Bar); // true
console.log(bar instanceof Object); // true, bar inherits from Object
console.log(bar.readA()); // 10 - Invoking inherited method in context of "bar" object
bar.writeA(20);
console.log(bar.readA()); // 20
console.log(bar.a); // 20 - Reading own porperty of "bar" object

readA and writeA are just regular JS functions with similar references to Function and Object as Bar function itself. These references are not shown for clarity. Important difference between them and Bar function is, that their prototypes are not of any use.

bar object has its own property a, because this property created every time Bar constructor is invoked. This behavior allows to produce different objects with their own property a, but inheriting "methods" from Bar.prototype.

Static methods

There is no such thing like static method in JavaScript spec at all, but this design pattern can easily be implemented by putting properties on a constructor function object itself, instead of its prototype object.

function Bar() {
  this.a = 10;
}

Bar.staticMethod = function () {
  // can not use "this" here
  return "I am static";
}

Bar.prototype.readA = function () {
  return this.a;
}

Bar.prototype.writeA = function (a) {
  this.a = a;
}

var bar = new Bar();

console.log(bar.staticMethod); // undefined, method can not be invoked on instance
console.log(Bar.staticMethod()); // "I am static"
console.log(bar.constructor.staticMethod()); // "I am static", method available on instance through inherited constructor property

Static methods of constructor are not accessible on instances, created with this consrtuctor, they are available on a constructor itself.

A lot of useful design patterns in JavaScript are implemented putting methods on a constructor function object, for example factory functions. Such a constructor can be used as a namespace or singleton.

Classical JavaScript inheritance and OOP

// Parent constructor
function Duck (name) {
  this.name = name;
};

// Parent method
Duck.prototype.quack = function () {
  return this.name + " Duck: Quack-quack!";
};

// Child constructor
function TalkingDuck (name) {
  // Call parent constructor with proper arguments
  Duck.call(this, name); // This is often forgotten
}

// Inheritance
TalkingDuck.prototype = Object.create(Duck.prototype);
TalkingDuck.prototype.constructor = TalkingDuck; // This is often forgotten

// Method overload
TalkingDuck.prototype.quack = function () {
  // Call parent method
  return Duck.prototype.quack.call(this) + " My name is " + this.name;
};

// Object instantiation
var donald = new TalkingDuck("Donald");
console.log(donald.quack()); // "Donald Duck: Quack-quack! My name is Donald"

console.log(donald.__proto__ === TalkingDuck.prototype); // true
console.log(TalkingDuck.prototype.__proto__ === Duck.prototype); // true, result of invoking Object.create()
console.log(Duck.prototype.__proto__ === Object.prototype); // true
console.log(donald.quack === TalkingDuck.prototype.quack); // true, method found by prototype chain
console.log(donald instanceof TalkingDuck); // true
console.log(donald instanceof Duck); // true
console.log(donald.name); // "Donald", reading own property

Prototype chain of donald is donald -> TalkingDuck.prototype -> Duck.prototype -> Object.prototype. Function.prototype does not taking part in this chain, since donald is not a function.

Note that name is own property of donald, though it is created with Duck constructor. This is because Duck constructor is invoked with Duck.call(this, name), where this points to newly created object inside TalkingDuck constructor and then passed down as an invocation context to Duck constructor. See MDN call() reference

Prototype pollution

Prototype pollution is changing properties of objects taking part in a prototype chain, affecting all other existing instances. Usually these objects are of type SomeConstructor.prototype. Setting properties values on them makes these properties visible for all instances of SomeConstructor and its descendant constructors resulting in unpredictable or undesired behaviours.

Another case is an incorrect constructor function design. The rule of thumb, is not to put value holding properties on a constructor prototype object, but only initialize them inside constructor function body. Constructor prototype should contain only methods.

Wrong!

function Collection(){}

// Shared between instances, - wrong
Collection.prototype.elements = [];

Collection.prototype.add = function (x) {
  // Each instance adds values to the same array
  this.elements.push(x);
};

It should be

function Collection(){
  // Each instance gets its own array, - good
  this.elements = [];
}

// No changes in code, but result will be different
Collection.prototype.add = function (x) {
  this.elements.push(x);
};

Next case of prototype pollution is any kind of modification of built-in constructor prototype object, for example Object.prototype or Function.prototype. Sometimes it is really tempting and some frameworks do it as a feature, but now it is considered strongly as an anti-pattern by JS community.

Factory functions

Factory function in JavaScript is not a language feature, it is a pattern. Many patterns in JavaScript are available just because of the power of prototype-based inheritance.

Factory function is very popular and powerful JavaScript OOP pattern. There are hundreds of implementations of it in many libraries and frameworks.

Factory function pattern is simple, it allows you create JavaScript objects and establish prototypal inheritance chain, but without usage of constructor function, using factory function instead. Factory functions can use constructors behind the scenes.

Node.js http.createServer is a typical example of factory function. It returns a new instance of http.Server class. New object is created without using constructor function and new keyword.

// Load the http module
var http = require('http');

// Call factory function to construct a new instance
var server = http.createServer(function (request, response) {
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.end("Hello World\n");
});

// server is an instance of http.Server class
console.log(server instanceof http.Server); // true

// Call listen method inherited from http.Server class
server.listen(8000);

console.log(server.hasOwnProperty('listen')); // false

Factory function can result in prototype chain similar to what we get using constructors. It is also can be implemented without usage of constructors, replacing them with some sort of init function, but still resulting in a prototype chain of objects. In later case instanceof operator will not work, and developer may need to use Duck typing technique to detect object type.

Mixins

Mixin is another popular and powerful JavaScript pattern. It is also kind of inheritance but not using prototypal inheritance chain.

Main goals of using mixins are multiple inheritance and performance boost.

Mixin is a function, which borrows behaviours from one or more donor objects to target object. Technically, it consists of copying all methods and properties from donors to target object, allowing it to achieve similar behaviour.

The most important part here, is that methods, copied from donor are not copied or cloned itself. Function objects are not duplicated. Mixin function only copies references to them. All resulting mixed instances refer to the same set of functions.

Multiple inheritance is achieved by mixing behaviours from several objects into one.

Performance boost is achieved by the fact, that prototype chain lookups are expensive, and when all methods from donors are reside directly on a child, it skips lookup and accelerates invocation.

Mixin function holds intact __proto__ property of target object.

instanceof operator will not work.

Reading

• MDN Introduction to Object-Oriented JavaScript
• MDN Inheritance and the prototype chain
• MDN Function
• MDN Object
• Dr. Axel Rauschmayer blog
• Juriy Zaytsev (kangax) blog
• Dmitry Soshnikov blog
• Addy Osmani "Learning JavaScript Design Patterns"