Inheritance in JavaScript

With most of the gory details of OOJS now explained, this article shows how to create "child" object classes (constructors) that inherit features from their "parent" classes. In addition, we present some advice on when and where you might use OOJS.

Prototypal inheritance

So far we have seen some inheritance in action — we have seen how prototype chains work, and how members are inherited going up a chain. But mostly this has involved built-in browser functions. How do we create an object in JavaScript that inherits from another object?

As mentioned earlier in the course, some people think JavaScript is not a true object-oriented language. In "classic OO" languages, you tend to define class objects of some kind, and you can then simply define which classes inherit from which other classes (see C++ inheritance for some simple examples). JavaScript uses a different system — "inheriting" objects do not have functionality copied over to them, instead the functionality they inherit is linked to via the prototype chain (often referred to as prototypal inheritance).

Let's explore how to do this with a concrete example.

Getting started

First of all, make yourself a local copy of our oojs-class-inheritance-start.html file (see it running live also). Inside here you'll find the same Person() constructor example that we've been using all the way through the module, with a slight difference — we've defined only the properties inside the constructor:

function Person(first, last, age, gender, interests) {
  this.name = {
    first,
    last
  };
  this.age = age;
  this.gender = gender;
  this.interests = interests;
};

The methods are all defined on the constructor's prototype. For example:

Person.prototype.greeting = function() {
  alert('Hi! I\'m ' + this.name.first + '.');
};

Say we wanted to create a Teacher class, like the one we described in our initial object-oriented definition, which inherits all the members from Person, but also includes:

1. A new property, subject — this will contain the subject the teacher teaches.
2. An updated greeting() method, which sounds a bit more formal than the standard greeting() method — more suitable for a teacher addressing some students at school.

Defining a Teacher() constructor function

The first thing we need to do is create a Teacher() constructor — add the following below the existing code:

function Teacher(first, last, age, gender, interests, subject) {
  Person.call(this, first, last, age, gender, interests);
  this.subject = subject;
}

This looks similar to the Person constructor in many ways, but there is something strange here that we've not seen before — the call() function. This function basically allows you to call a function defined somewhere else, but in the current context. The first parameter specifies the value of this that you want to use when running the function, and the other parameters are those that should be passed to the function when it is invoked.

We want the Teacher() constructor to take the same parameters as the Person() constructor it is inheriting from, so we specify them all as parameters in the call() invocation.

The last line inside the constructor simply defines the new subject property that teachers are going to have, which generic people don't have.

As a note, we could have simply done this:

function Teacher(first, last, age, gender, interests, subject) {
  this.name = {
    first,
    last
  };
  this.age = age;
  this.gender = gender;
  this.interests = interests;
  this.subject = subject;
}

But this is just redefining the properties anew, not inheriting them from Person(), so it defeats the point of what we are trying to do. It also takes more lines of code.

Inheriting from a constructor with no parameters

Note that if the constructor you are inheriting from doesn't take its property values from parameters, you don't need to specify them as additional arguments in call(). So, for example,  if you had something really simple like this:

function Brick() {
  this.width = 10;
  this.height = 20;
}

You could inherit the width and height properties by doing this (as well as the other steps described below, of course):

function BlueGlassBrick() {
  Brick.call(this);
  this.opacity = 0.5;
  this.color = 'blue';
}

Note that we've only specified this inside call() — no other parameters are required as we are not inheriting any parameters, only properties.

Setting Teacher()'s prototype and constructor reference

All is good so far, but we have a problem. We have defined a new constructor, and it has a prototype property, which by default just contains a reference to the constructor function itself. It does not contain the methods of the Person constructor's prototype property. To see this, enter Object.getOwnPropertyNames(Teacher.prototype) into either the text input field or your JavaScript console. Then enter it again, replacing Teacher with Person. Nor does the new constructor inherit those methods. To see this, compare the outputs of Person.prototype.greeting and Teacher.prototype.greeting. We need to get Teacher() to inherit the methods defined on Person()'s prototype. So how do we do that?

1. Add the following line below your previous addition:

   Teacher.prototype = Object.create(Person.prototype);

   Here our friend create() comes to the rescue again. In this case we are using it to create a new object and make it the value of Teacher.prototype. The new object has Person.prototype as its prototype and will therefore inherit, if and when needed, all the methods available on Person.prototype.
2. We need to do one more thing before we move on. After adding the last line, Teacher.prototype's constructor property is now equal to Person(), because we just set Teacher.prototype to reference an object that inherits its properties from Person.prototype! Try saving your code, loading the page in a browser, and entering Teacher.prototype.constructor into the console to verify.
3. This can become a problem, so we need to set this right. You can do so by going back to your source code and adding the following line at the bottom:

   Teacher.prototype.constructor = Teacher;

4. Now if you save and refresh, entering Teacher.prototype.constructor should return Teacher(), as desired, plus we are now inheriting from Person()!

Giving Teacher() a new greeting() function

To finish off our code, we need to define a new greeting() function on the Teacher() constructor.

The easiest way to do this is to define it on Teacher()'s prototype — add the following at the bottom of your code:

Teacher.prototype.greeting = function() {
  var prefix;
  if (this.gender === 'male' || this.gender === 'Male' || this.gender === 'm' || this.gender === 'M') {
    prefix = 'Mr.';
  } else if (this.gender === 'female' || this.gender === 'Female' || this.gender === 'f' || this.gender === 'F') {
    prefix = 'Mrs.';
  } else {
    prefix = 'Mx.';
  }
  alert('Hello. My name is ' + prefix + ' ' + this.name.last + ', and I teach ' + this.subject + '.');
};

This alerts the teacher's greeting, which also uses an appropriate name prefix for their gender, worked out using a conditional statement.

Trying the example out

Now that you've entered all the code, try creating an object instance from Teacher() by putting the following at the bottom of your JavaScript (or something similar of your choosing):

var teacher1 = new Teacher('Dave', 'Griffiths', 31, 'male', ['football', 'cookery'], 'mathematics');

Now save and refresh, and try accessing the properties and methods of your new teacher1 object, for example:

teacher1.name.first;
teacher1.interests[0];
teacher1.bio();
teacher1.subject;
teacher1.greeting();
teacher1.farewell();

These should all work just fine. The queries on lines 1, 2, 3, and 6 access members inherited from the generic Person() constructor (class). The query on line 4 accesses a member that is available only on the more specialized Teacher() constructor (class). The query on line 5 would have accessed a member inherited from Person(), except for the fact that Teacher() has its own member with the same name, so the query accesses that member.

The technique we covered here is not the only way to create inheriting classes in JavaScript, but it works OK, and it gives you a good idea about how to implement inheritance in JavaScript.

You might also be interested in checking out some of the new ECMAScript features that allow us to do inheritance more cleanly in JavaScript (see Classes). We didn't cover those here, as they are not yet supported very widely across browsers. All the other code constructs we discussed in this set of articles are supported as far back as IE9 or earlier, and there are ways to achieve earlier support than that.

A common way is to use a JavaScript library — most of the popular options have an easy set of functionality available for doing inheritance more easily and quickly. CoffeeScript for example provides class, extends, etc.

A further exercise

In our OOP theory section, we also included a Student class as a concept, which inherits all the features of Person, and also has a different greeting() method from Person that is much more informal than the Teacher's greeting. Have a look at what the student's greeting looks like in that section, and try implementing your own Student() constructor that inherits all the features of Person(), and implements the different greeting() function.

Object member summary

To summarize, you've basically got three types of property/method to worry about:

1. Those defined inside a constructor function that are given to object instances. These are fairly easy to spot — in your own custom code, they are the members defined inside a constructor using the this.x = x type lines; in built in browser code, they are the members only available to object instances (usually created by calling a constructor using the new keyword, e.g. var myInstance = new myConstructor()).
2. Those defined directly on the constructor themselves, that are available only on the constructor. These are commonly only available on built-in browser objects, and are recognized by being chained directly onto a constructor, not an instance. For example, Object.keys().
3. Those defined on a constructor's prototype, which are inherited by all instances and inheriting object classes. These include any member defined on a Constructor's prototype property, e.g. myConstructor.prototype.x().

If you are not sure which is which, don't worry about it just yet — you are still learning, and familiarity will come with practice.

When would you use inheritance in JavaScript?

Particularly after this last article, you might be thinking "woo, this is complicated". Well, you are right. Prototypes and inheritance represent some of the most complex aspects of JavaScript, but a lot of JavaScript's power and flexibility comes from its object structure and inheritance, and it is worth understanding how it works.

In a way, you use inheritance all the time. Whenever you use various features of a Web API , or methods/properties defined on a built-in browser object that you call on your strings, arrays, etc., you are implicitly using inheritance.

In terms of using inheritance in your own code, you probably won't use it often, especially to begin with, and in small projects. It is a waste of time to use objects and inheritance just for the sake of it when you don't need them. But as your code bases get larger, you are more likely to find a need for it. If you find yourself starting to create a number of objects that have similar features, then creating a generic object type to contain all the shared functionality and inheriting those features in more specialized object types can be convenient and useful.

When using inheritance, you are advised to not have too many levels of inheritance, and to keep careful track of where you define your methods and properties. It is possible to start writing code that temporarily modifies the prototypes of built-in browser objects, but you should not do this unless you have a really good reason. Too much inheritance can lead to endless confusion, and endless pain when you try to debug such code.

Ultimately, objects are just another form of code reuse, like functions or loops, with their own specific roles and advantages. If you find yourself creating a bunch of related variables and functions and want to track them all together and package them neatly, an object is a good idea. Objects are also very useful when you want to pass a collection of data from one place to another. Both of these things can be achieved without use of constructors or inheritance. If you only need a single instance of an object, then you are probably better off just using an object literal, and you certainly don't need inheritance.

Summary

This article has covered the remainder of the core OOJS theory and syntax that we think you should know now. At this point you should understand JavaScript object and OOP basics, prototypes and prototypal inheritance, how to create classes (constructors) and object instances, add features to classes, and create subclasses that inherit from other classes.

In the next article we'll have a look at how to work with JavaScript Object Notation (JSON), a common data exchange format written using JavaScript objects.

See also

• ObjectPlayground.com — A really useful interactive learning site for learning about objects.
• Secrets of the JavaScript Ninja, Chapter 6 — A good book on advanced JavaScript concepts and techniques, by John Resig and Bear Bibeault. Chapter 6 covers aspects of prototypes and inheritance really well; you can probably track down a print or online copy fairly easily.
• You Don't Know JS: this & Object Prototypes — Part of Kyle Simpson's excellent series of JavaScript manuals, Chapter 5 in particular looks at prototypes in much more detail than we do here. We've presented a simplified view in this series of articles aimed at beginners, whereas Kyle goes into great depth and provides a more complex but more accurate picture.