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Object-orientation and inheritance in JavaScript: a comprehensive explanation

Posted on •    16 mins read

Let’s talk about object-orientation and inheritance in JavaScript.

The good news is that it’s actually quite simple, but the bad news is that it works completely different than object-orientation in languages like C++, Java, Ruby, Python or PHP, making it not-quite-so simple to understand.

But fear not, we are going to take it step by step.

Blueprints versus finger-pointing

Let’s start by looking at how "typical" object-oriented languages actually create objects.

We are going to talk about an object called myCar. myCar is our bits-and-bytes representation of an incredibly simplified real world car. It could have attributes like color and weight, and methods like drive and honk.

In a "real" application, myCar could be used to represent the car in a game which is driven by the player of that game – but we are going to completely ignore the context of this object, because we are going to talk about the nature and usage of this object in a more abstract way.

If you would want to use this myCar object in, say, Java, you need to define the blueprint of this specific object first – this is what Java and most other object-oriented languages call a class.

If you want to create the object myCar, you tell Java to "build a new object after the specification that is laid out in the class Car".

The newly built object shares certain aspects with its blueprint. If you call the method honk on your object, like so:

myCar.honk();

the Java interpreter will go to the class of myCar and look up which code it actually needs to execute, which is defined in the honk method of class Car.

Ok, nothing shockingly new here. Enter JavaScript.

A classless society

JavaScript does not have classes. But as in other languages, we would like to tell the interpreter that it should built our myCar object following a certain pattern or schema or blueprint – it would be quite tedious to create every car object from scratch, "manually" giving it the attributes and methods it needs every time we build it.

If we were to create 30 car objects based on the Car class in Java, this object-class relationship provides us with 30 cars that are able to drive and honk without us having to write 30 drive and honk methods.

How is this achieved in JavaScript? Instead of an object-class relationship, there is an object-object relationship.

Where in Java our myCar, asked to honk, says "go look at this class over there, which is my blueprint, to find the code you need", JavaScript says "go look at that other object over there, which is my prototype, it has the code you are looking for".

Building objects via an object-object relationship is called Prototype-based programming, versus Class-based programming used in more traditional languages like Java.

Both are perfectly valid implementations of the object-oriented programming paradigm – it’s just two different approaches.

Creating objects

Let’s dive into code a bit, shall we? How could we set up our code in order to allow us to create our myCar object, ending up with an object that is a Car and can therefore honk and drive?

Well, in the most simple sense, we can create our object completely from scratch, or ex nihilo if you prefer the boaster expression.

It works like this:

var myCar = {}

myCar.honk = function() {
  console.log("honk honk");
}

myCar.drive = function() {
  console.log("vrooom...");
}

This gives us an object called myCar that is able to honk and drive:

myCar.honk()  // outputs "honk honk"
myCar.drive() // outputs "vrooom..."

However, if we were to create 30 cars this way, we would end up defining the honk and drive behaviour of every single one, something we said we want to avoid.

In real life, if we made a living out of creating, say, pencils, and we don’t want to create every pencil individually by hand, then we would consider building a pencil-making machine, and have this machine create the pencils for us.

After all, that’s what we implicitly do in a class-based language like Java – by defining a class Car, we get the car-maker for free:

Car myCar = new Car();

will built the myCar object for us based on the Car blueprint. Using the new keyword does all the magic for us.

JavaScript, however, leaves the responsibility of building an object creator to us. Furthermore, it gives us a lot of freedom regarding the way we actually build our objects.

In the most simple case, we can write a function which creates "plain" objects that are exactly like our "ex nihilo" object, and that don’t really share any behaviour – they just happen to roll out of the factory with the same behaviour copied onto every single one, if you want so.

Or, we can write a special kind of function that not only creates our objects, but also does some behind-the-scenes magic which links the created objects with their creator. This allows for a true sharing of behaviour: functions that are available on all created objects point to a single implementation. If this function implementation changes after objects have been created, which is possible in JavaScript, the behaviour of all objects sharing the function will change accordingly.

Let’s examine all possible ways of creating objects in detail.

Using a simple function to create plain objects

In our first example, we created a plain myCar object out of thin air – we can simply wrap the creation code into a function, which gives us a very basic object creator:

function makeCar() {
  var newCar = {}
  newCar.honk = function() {
    console.log("honk honk");
  }
}

For the sake of brevity, the drive function has been omitted.

We can then use this function to mass-produce cars:

function makeCar() {
  var newCar = {}
  newCar.honk = function() {
    console.log("honk honk");
  }
  return newCar;
}

myCar1 = makeCar();
myCar2 = makeCar();
myCar3 = makeCar();

One downside of this approach is efficiency: for every myCar object that is created, a new honk function is created and attached – creating 1,000 objects means that the JavaScript interpreter has to allocate memory for 1,000 functions, although they all implement the same behaviour. This results in an unnecessarily high memory footprint of the application.

Secondly, this approach deprives us of some interesting opportunities. These myCar objects don’t share anything – they were built by the same creator function, but are completely independent from each other.

It’s really like with real cars from a real car factory: They all look the same, but once they leave the assembly line, they are totally independent. If the manufacturer should decide that pushing the horn on already produced cars should result in a different type of honk, all cars would have to be returned to the factory and modified.

In the virtual universe of JavaScript, we are not bound to such limits. By creating objects in a more sophisticated way, we are able to magically change the behaviour of all created objects at once.

Using a constructor to create objects

In JavaScript, the entities that create objects with shared behaviour are functions which are called in a special way. These special functions are called constructors.

Let’s create a constructor for cars. We are going to call this function Car, with a capital C, which is common practice to indicate that this function is a constructor.

Because we are going to encounter two new concepts that are both necessary for shared object behaviour to work, we are going to approach the final solution in two steps.

Step one is to recreate the previous solution (where a common function spilled out independent car objects), but this time using a constructor:

function Car() {
  this.honk = function() {
    console.log("honk honk");
  }
}

When this function is called using the new keyword, like so:

var myCar = new Car();

it implicitly returns a newly created object with the honk function attached.

Using this and new makes the explicit creation and return of the new object unnecessary – it is created and returned "behind the scenes" (i.e., the new keyword is what creates the new, "invisible" object, and secretly passes it to the Car function as its this variable).

You can think of the mechanism at work a bit like in this pseudo-code:

// Pseudo-code, for illustration only!

function Car(this) {
  this.honk = function() {
    console.log("honk honk");
  }
  return this;
}

var newObject = {}
var myCar = Car(newObject);

As said, this is more or less like our previous solution – we don’t have to create every car object manually, but we still cannot modify the honk behaviour only once and have this change reflected in all created cars.

But we laid the first cornerstone for it. By using a constructor, all objects received a special property that links them to their constructor:

function Car() {
  this.honk = function() {
    console.log("honk honk");
  }
}

var myCar1 = new Car();
var myCar2 = new Car();

console.log(myCar1.constructor); // outputs [Function: Car]
console.log(myCar2.constructor); // outputs [Function: Car]

All created myCars are linked to the Car constructor. This is what actually makes them a class of related objects, and not just a bunch of objects that happen to have similar names and identical functions.

Now we have finally reached the moment to get back to the mysterious prototype we talked about in the introduction.

Using prototyping to efficiently share behaviour between objects

As stated there, while in class-based programming the class is the place to put functions that all objects will share, in prototype-based programming, the place to put these functions is the object which acts as the prototype for our objects at hand.

But where is the object that is the prototype of our myCar objects – we didn’t create one!

It has been implicitly created for us, and is assigned to the

Car.prototype

property (in case you wondered, JavaScript functions are objects that have properties, too).

Here is the key to sharing functions between objects: Whenever we call a function on an object, the JavaScript interpreter tries to find that function within the queried object. But if it doesn’t find the function within the object itself, it asks the object for the pointer to it’s prototype, then goes to the prototype, and asks for the function there. If it is found, it is then executed.

This means that we can create myCar objects without any functions, create the honk function in their prototype, and end up having myCar objects that know how to honk – because everytime the interpreter tries to execute the honk function on one of the myCar objects, it will be redirected to the prototype, and execute the honk function which is defined there.

Here is how this setup can be achieved:

function Car() {}

Car.prototype.honk = function() {
  console.log("honk honk");
}

var myCar1 = new Car();
var myCar2 = new Car();

myCar1.honk(); // executes Car.prototype.honk() and outputs "honk honk"
myCar2.honk(); // executes Car.prototype.honk() and outputs "honk honk"

Our constructor is now empty, because for our very simple cars, no additional setup is necessary.

Because both myCars are created through this constructor, their prototype points to Car.prototype – executing myCar1.honk() results in Car.prototype.honk() being executed.

Let’s see what this enables us to do. In JavaScript, objects can be changed at runtime. This holds true for prototypes, too. Which is why we can change the honk behaviour of all our cars even after they have been created:

function Car() {}

Car.prototype.honk = function() {
  console.log("honk honk");
}

var myCar1 = new Car();
var myCar2 = new Car();

myCar1.honk(); // executes Car.prototype.honk() and outputs "honk honk"
myCar2.honk(); // executes Car.prototype.honk() and outputs "honk honk"

Car.prototype.honk = function() {
  console.log("meep meep");
}

myCar1.honk(); // executes Car.prototype.honk() and outputs "meep meep"
myCar2.honk(); // executes Car.prototype.honk() and outputs "meep meep"

Of course, we can also add additional functions at runtime:

function Car() {}

Car.prototype.honk = function() {
  console.log("honk honk");
}

var myCar1 = new Car();
var myCar2 = new Car();

Car.prototype.drive = function() {
  console.log("vrooom...");
}

myCar1.drive(); // executes Car.prototype.drive() and outputs "vrooom..."
myCar2.drive(); // executes Car.prototype.drive() and outputs "vrooom..."

But we could even decide to treat only one of our cars differently:

function Car() {}

Car.prototype.honk = function() {
  console.log("honk honk");
}

var myCar1 = new Car();
var myCar2 = new Car();

myCar1.honk(); // executes Car.prototype.honk() and outputs "honk honk"
myCar2.honk(); // executes Car.prototype.honk() and outputs "honk honk"

myCar2.honk = function() {
  console.log("meep meep");
}

myCar1.honk(); // executes Car.prototype.honk() and outputs "honk honk"
myCar2.honk(); // executes myCar2.honk() and outputs "meep meep"

It’s important to understand what happens behind the scenes in this example. As we have seen, when calling a function on an object, the interpreter follows a certain path to find the actual location of that function.

While for myCar1, there still is no honk function within that object itself, that no longer holds true for myCar2. When the interpreter calls myCar2.honk, there now is a function within myCar2 itself. Therefore, the interpreter no longer follows the path to the prototype of myCar2, and executes the function within myCar2 instead.

That’s one of the major differences to class-based programming: while objects are relatively "rigid" e.g. in Java, where the structure of an object cannot be changed at runtime, in JavaScript, the prototype-based approach links objects of a certain class more loosely together, which allows to change the structure of objects at any time.

Also, note how sharing functions through the constructor’s prototype is way more efficient than creating objects that all carry their own functions, even if they are identical. As previously stated, the engine doesn’t know that these functions are meant to be identical, and it has to allocate memory for every function in every object. This is no longer true when sharing functions through a common prototype – the function in question is placed in memory exactly once, and no matter how many myCar objects you create, they don’t carry the function themselves, they only refer to their constructor, in whose prototype the function is found.

To give you an idea of what this difference can mean, here is a very simple comparison. The first example creates 1,000,000 objects that all have the function directly attached to them: 

var C = function() {
  this.f = function(foo) {
    console.log(foo);
  }
}

var a = [];
for (var i = 0; i < 1000000; i++) {
  a.push(new C());
}

In Google Chrome, this results in a heap snapshot size of 328 MB. Here is the same example, but now the function is shared through the constructor's prototype:

var C = function() {}

C.prototype.f = function(foo) {
  console.log(foo);
}

var a = [];
for (var i = 0; i < 1000000; i++) {
  a.push(new C());
}

This time, the size of the heap snapshot is only 17 MB, i.e., only about 5% of the non-efficient solution.

Object-orientation, prototyping, and inheritance

So far, we haven't talked about inheritance in JavaScript, so let's do this now.

It's useful to share behaviour between a certain class of objects, but there are cases where we would like to share behaviour between different, but similar classes of objects.

Imagine our virtual world not only had cars, but also bikes. Both drive, but where a car has a horn, a bike has a bell.

Being able to drive makes both objects vehicles, but not sharing the honk and ring distinguishes them.

We could illustrate their shared and local behaviour as well as their relationship to each other as follows:

         Vehicle
         > drive

            |
 ----------------------
 |                    |

Car                 Bike
> honk              > ring

Designing this relationship in a class-based language like Java is straightforward: We would define a class Vehicle with a method drive, and two classes Car and Bike which both extend the Vehicle class, and implement a honk and a ring method, respectively.

This would make the car as well as bike objects inherit the drive behaviour through the inheritance of their classes.

How does this work in JavaScript, where we don't have classes, but prototypes?

Let's look at an example first, and then dissect it. To keep the code short for now, let's only start with a car that inherits from a vehicle:

function Vehicle() {}

Vehicle.prototype.drive = function () {
  console.log("vrooom...");
}


function Car() {}

Car.prototype = new Vehicle();

Car.prototype.honk = function() {
  console.log("honk honk");
}


var myCar = new Car();

myCar.honk()  // outputs "honk honk"
myCar.drive()  // outputs "vrooom..."

In JavaScript, inheritance runs through a chain of prototypes.

The prototype of the Car constructor is set to a newly created vehicle object, which establishes the link structure that allows the interpreter to look for methods in parent objects.

The prototype of the Vehicle constructor has a function drive. Here is what happens when the myCar object is asked to drive():

  • The interpreter looks for a drive method within the myCar object, which does not exist
  • The interpreter then asks the myCar object for its prototype, which is the prototype of its constructor Car
  • When looking at Car.prototype, the interpreter sees a vehicle object which has a function honk attached, but no drive function
  • Thus, the interpreter now asks this vehicle object for its prototype, which is the prototype of its constructor Vehicle
  • When looking at Vehicle.prototype, the interpreter sees an object which has a drive function attached - the interpreter now knows which code implements the myCar.drive() behaviour, and executes it

A classless society, revisited

We just learned how to emulate traditional (or classical) inheritance in JavaScript. This understanding is needed to successfully unlearn it and leave it behind, in order to embrace the idea that in JavaScript, you really don't need classes at all, and you therefore don't need to emulate them - plus, that's really a lot of code to express the prototypical idea of "go look at that other object over there, it has the code you are looking for", isn't it?

It was Douglas Crockford who came up with a very clever solution, which allows to let objects directly inherit from each other, without the need for all the boilerplate code presented in the previous example. The solution is a native part of JavaScript by now - it's the Object.create() function, and it works like this: 

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

We learned enough now to understand what's going on. Let's analyze an example: 

var vehicle = {};
vehicle.drive = function () {
  console.log("vrooom...");
}

var car = Object.create(vehicle);
car.honk = function() {
  console.log("honk honk");
}

var myCar = Object.create(car);

myCar.honk()  // outputs "honk honk"
myCar.drive()  // outputs "vrooom..."

While being significantly more concise and expressive, this code achieves exactly the same behaviour, without the need to write dedicated constructors and attaching functions to their prototype. As you can see, Object.create() handles both behind the scenes, on the fly. A temporary constructor is created, its prototype is set to the object that serves as the role model for our new object, and a new object is created from this setup.

Conceptually, this is really the same as in the previous example where we defined that Car.prototype shall be new Vehicle();.

But wait! We created the functions drive and honk within our objects, not on their prototypes - that's memory-inefficient!

Well, in this case, it's actually not. Let's see why: 

var vehicle = {};
vehicle.drive = function () {
  console.log("vrooom...");
}

var car = Object.create(vehicle);
car.honk = function() {
  console.log("honk honk");
}

var myVehicle = Object.create(vehicle);
var myCar1 = Object.create(car);
var myCar2 = Object.create(car);

myCar1.honk()  // outputs "honk honk"
myCar2.honk()  // outputs "honk honk"

myVehicle.drive()  // outputs "vrooom..."
myCar1.drive()     // outputs "vrooom..."
myCar2.drive()     // outputs "vrooom..."

We have now created a total of 5 objects, but how often do the honk and drive methods exist in memory? Well, how often have they been defined? Just once - and therefore, this solution is basically as efficient as the one where we built the inheritance manually. Let's look at the numbers: 

c = {};
c.f = function(foo) {
  console.log(foo);
}

var a = [];
for (var i = 0; i < 1000000; i++) {
  a.push(Object.create(c));
}

Turns out, it's not exactly identical - we end up with a heap snapshot size of 40 MB, thus there seems to be some overhead involved. However, in exchange for much better code, this is probably more than worth it.