Object prototypes

Prototypes are the mechanism by which JavaScript objects inherit features from one another. In this article, we explain what a prototype is, how prototype chains work, and how a prototype for an object can be set.

Prerequisites: Understanding JavaScript functions, familiarity with JavaScript basics (see First steps and Building blocks), and OOJS basics (see Introduction to objects).
Objective: To understand JavaScript object prototypes, how prototype chains work, and how to set the prototype of an object.

The prototype chain

In the browser's console, try creating an object literal:

const myObject = {
  city: 'Madrid',
  greet() {
    console.log(`Greetings from ${this.city}`);
  }
}

myObject.greet(); // Greetings from Madrid

This is an object with one data property, city, and one method, greet(). If you type the object's name followed by a period into the console, like myObject., then the console will pop up a list of all the properties available to this object. You'll see that as well as city and greet, there are lots of other properties!

__defineGetter__
__defineSetter__
__lookupGetter__
__lookupSetter__
__proto__
city
constructor
greet
hasOwnProperty
isPrototypeOf
propertyIsEnumerable
toLocaleString
toString
toValueOf

Try accessing one of them:

myObject.toString(); // "[object Object]"

It works (even if it's not obvious what toString() does).

What are these extra properties, and where do they come from?

Every object in JavaScript has a built-in property, which is called its prototype. The prototype is itself an object, so the prototype will have its own prototype, making what's called a prototype chain. The chain ends when we reach a prototype that has null for its own prototype.

Note: The property of an object that points to its prototype is not called prototype. Its name is not standard, but in practice all browsers use __proto__. The standard way to access an object's prototype is the Object.getPrototypeOf() method.

When you try to access a property of an object: if the property can't be found in the object itself, the prototype is searched for the property. If the property still can't be found, then the prototype's prototype is searched, and so on until either the property is found, or the end of the chain is reached, in which case undefined is returned.

So when we call myObject.toString(), the browser:

  • looks for toString in myObject
  • can't find it there, so looks in the prototype object of myObject for toString
  • finds it there, and calls it.

What is the prototype for myObject? To find out, we can use the function Object.getPrototypeOf():

Object.getPrototypeOf(myObject); // Object {...}

This is an object called Object.prototype, and it is the most basic prototype, that all objects have by default. The prototype of Object.prototype is null, so it's at the end of the prototype chain:

Prototype chain for myObject

The prototype of an object is not always Object.prototype. Try this:

const myDate = new Date();
let object = myDate;

do {
  object = Object.getPrototypeOf(object);
  console.log(object);
} while (object);

// Date.prototype
// Object {...}
// null

This code creates a Date object, then walks up the prototype chain, logging the prototypes. It shows us that the prototype of myDate is a Date.prototype object, and the prototype of that is Object.prototype.

Prototype chain for myDate

In fact, when you call familiar methods, like myDate2.getMonth(), you are calling a method that's defined on Date.prototype.

Shadowing properties

What happens if you define a property in an object, when a property with the same name is defined in the object's prototype? Let's see:

const myDate = new Date(1995, 11, 17);

console.log(myDate.getYear()); // 95

myDate.getYear = function() {
  console.log('something else!')
};

myDate.getYear(); // 'something else!'

This should be predictable, given the description of the prototype chain. When we call getYear() the browser first looks in myDate for a property with that name, and only checks the prototype if myDate does not define it. So when we add getYear() to myDate, then the version in myDate is called.

This is called "shadowing" the property.

Setting a prototype

There are various ways of setting an object's prototype in JavaScript, and here we'll describe two: Object.create() and constructors.

Using Object.create

The Object.create() method creates a new object and allows you to specify an object that will be used as the new object's prototype.

Here's an example:

const personPrototype = {
  greet() {
    console.log('hello!');
  }
}

const carl = Object.create(personPrototype);
carl.greet();  // hello!

Here we create an object personPrototype, which has a greet() method. We then use Object.create() to create a new object with personPrototype as its prototype. Now we can call greet() on the new object, and the prototype provides its implementation.

Using a constructor

In JavaScript, all functions have a property named prototype. When you call a function as a constructor, this property is set as the prototype of the newly constructed object (by convention, in the property named __proto__).

So if we set the prototype of a constructor, we can ensure that all objects created with that constructor are given that prototype:

const personPrototype = {
  greet() {
    console.log(`hello, my name is ${this.name}!`);
  }
}

function Person(name) {
  this.name = name;
}

Person.prototype = personPrototype;
Person.prototype.constructor = Person;

Here we create:

  • an object personPrototype, which has a greet() method
  • a Person() constructor function which initializes the name of the person to create.

We then set the Person function's prototype property to point to personPrototype.

The last line (Person.prototype.constructor = Person;) sets the prototype's constructor property to the function used to create Person objects. This is required because after setting Person.prototype = personPrototype; the property points to the constructor for the personPrototype, which is Object rather than Person (because personPrototype was constructed as an object literal).

After this code, objects created using Person() will get personPrototype as their prototype.

const reuben = new Person('Reuben');
reuben.greet(); // hello, my name is Reuben!

This also explains why we said earlier that the prototype of myDate is called Date.prototype: it's the prototype property of the Date constructor.

Own properties

The objects we create using the Person constructor above have two properties:

  • a name property, which is set in the constructor, so it appears directly on Person objects
  • a greet() method, which is set in the prototype.

It's common to see this pattern, in which methods are defined on the prototype, but data properties are defined in the constructor. That's because methods are usually the same for every object we create, while we often want each object to have its own value for its data properties (just as here where every person has a different name).

Properties that are defined directly in the object, like name here, are called own properties, and you can check whether a property is an own property using the static Object.hasOwn() method:

const irma = new Person('Irma');

console.log(Object.hasOwn(irma, 'name')); // true
console.log(Object.hasOwn(irma, 'greet')); // false

Note: You can also use the non-static Object.hasOwnProperty() method here, but we recommend that you use Object.hasOwn() if you can.

Prototypes and inheritance

Prototypes are a powerful and very flexible feature of JavaScript, making it possible to reuse code and combine objects.

In particular they support a version of inheritance. Inheritance is a feature of object-oriented programming languages that lets programmers express the idea that some objects in a system are more specialized versions of other objects.

For example, if we're modeling a school, we might have professors and students: they are both people, so have some features in common (for example, they both have names), but each might add extra features (for example, professors have a subject that they teach), or might implement the same feature in different ways. In an OOP system we might say that professors and students both inherit from people.

You can see how in JavaScript, if Professor and Student objects can have Person prototypes, then they can inherit the common properties, while adding and redefining those properties which need to differ.

In the next article we'll discuss inheritance along with the other main features of object-oriented programming languages, and see how JavaScript supports them.

Summary

This article has covered JavaScript object prototypes, including how prototype object chains allow objects to inherit features from one another, the prototype property and how it can be used to add methods to constructors, and other related topics.

In the next article we'll look at the concepts underlying object-oriented programming.

In this module