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Objects, [[Prototype]] and Prototypal Inheritance in JavaScript

alexdevero profile image Alex Devero Originally published at blog.alexdevero.com ・9 min read

Prototypal inheritance is one of the topics in programming every developer should know. This tutorial will help you learn what prototypal inheritance is, how it works and how to use it. You will learn about [[Prototype]], __proto__, how to change prototypes and much much more.

Creating, inheriting and re-usability

Knowing how to create objects is useful. It can help you do more things, often in better way. However, creating objects from scratch may not always be the best thing to do. The problem is that this practice can lead to repeated code. What you can do instead is create a base object.

This base object will contain universal properties and methods you may want in other objects. Later, let's say you want to create an object that will use any of these properties or methods. You don't have to write all those properties and methods from scratch. Instead, you can let that new object inherit from the base object.

When you do this, that new object will be able to use any property and method that exists in the base object. This is not everything you can do, yet. You can also add additional properties and methods only to that new object. After this, the base object will still be the same.

That new object, however, will not only be able to use anything from the base object. It will also be able to use anything new you've just added. This degree of re-usability can help you make your code much shorter, clearer and cleaner. This is how prototypal inheritance can help you.

The [[Prototype]] property

The fundamental part of prototypal inheritance is the [[Prototype]] property. This is a special hidden property that exists on every object in JavaScript. The value of this property is always either null or name of another object. When the value of [[Prototype]] is null it means that the object doesn't inherit from any other object.

When the value is a name of another object it means that the object's prototype references another object. Put simply, that object inherits from another object, whose name is specified in [[Prototype]]. When this happens, the inheriting object can use any property and method from the object it inherits from.

The [[Prototype]] property and prototypal inheritance

This is one of the things in JavaScript that can seem weird. Let's say you want to access some property in an object. If that property exists JavaScript will return it. In case of a method, it will call that method. What if the property you want to access, or method you want to call, doesn't exist on that object?

In that case, JavaScript will do something interesting. It will take a look at the value of [[Prototype]] property. If the value is not null, it will find the object this property refers to. When it finds it, it will take a look if that object contains the property you want to access, or method you want to call.

If the property exists, JavaScript will return its value. If the method exists, JavaScript will call it. This, in essence, is what prototypal inheritance is about. You can access "stuff" in one object even though you are working with a different object, if that different object inherits from the first object.

The proto, Object.setPrototypeOf() and Object.getPrototypeOf()

The [[Prototype]] property is hidden. However, there are ways that allow you to change its value. The one often used way to change prototype of an object is by using __proto__. One thing you should remember. The [[Prototype]] property and __proto__ are not the same thing.

The __proto__ is only a setter and getter for [[Prototype]] property. It allows you to work [[Prototype]] property. Another way to set [[Prototype]] is by using Object.setPrototypeOf() method. This is a more modern setter. More modern getter is Object.getPrototypeOf() method.

It is mostly due to overall support by browsers why __proto__ is more preferred than Object.setPrototypeOf()and Object.getPrototypeOf(). That said, using __proto__ is deprecated, and not recommended. What you should use instead is either Object.setPrototypeOf() or Object.getPrototypeOf().

Changing the prototype

You know about the __proto__, Object.setPrototypeOf() and Object.getPrototypeOf(). Now, let's take a look at how you can use them to change the prototype of an object. We will take a look at how to do this with both options, the Object.setPrototypeOf() as well as the __proto__.

The proto

Fist, the __proto__. When you want to change prototype with __proto__ you will assign a value. First, you need an object that should inherit from another object. You will access __proto__ of this object. After that, you will choose an object you want the inheriting object to inherit from.

The value of __proto__ will be a reference to that object you want to inherit from. You will use the name of that object as the value you assign to __proto__. That's it. Do this and you will successfully create prototypal inheritance between two objects.

// Create base object
const myBaseObj = {
  isAlive: true,
  canSpeak: true,
  sayHi() {
    return 'Hello!'
  }
}

// Create new object that will inherit from "myBaseObj"
// Add a couple of its own properties
const myNewObj = {
  canWalk: true,
  canRun: true
}

// Let "myNewObj" inherit from "myBaseObj"
// by setting "myNewObj" prototype to "myBaseObj"
myNewObj.__proto__ = myBaseObj

// Now "myNewObj" basically becomes
// const myNewObj = {
//   isAlive: true,
//   canSpeak: true,
//   sayHi() {
//     return 'Hello!'
//   },
//   canWalk: true,
//   canRun: true
// }

// Access inherited "isAlive" property on "myNewObj"
console.log('isAlive: ', myNewObj.isAlive)
// Output:
// 'isAlive: ' true

// Access inherited "canSpeak" property on "myNewObj"
console.log('canSpeak: ', myNewObj.canSpeak)
// Output:
// 'canSpeak: ' true

// Access own "canWalk" property on "myNewObj"
console.log('canWalk: ', myNewObj.canWalk)
// Output:
// 'canWalk: ' true

// Call inherited "sayHi" method on "myNewObj"
console.log(myNewObj.sayHi())
// Output:
// 'Hello!'

// Create another object that will also inherit from "myBaseObj"
const myAnotherObj = {
  canSleep: true
}

// Let "myAnotherObj" also inherit from "myBaseObj"
myAnotherObj.__proto__ = myBaseObj

// Now "myAnotherObj" basically becomes
// const myAnotherObj = {
//   isAlive: true,
//   canSpeak: true,
//   sayHi() {
//     return 'Hello!'
//   },
//   canSleep: true
// }

// Access inherited "isAlive" property on "myAnotherObj"
console.log('isAlive: ', myAnotherObj.isAlive)
// Output:
// 'isAlive: ' true

// Access inherited "canSpeak" property on "myAnotherObj"
console.log('canSpeak: ', myAnotherObj.canSpeak)
// Output:
// 'canSpeak: ' true

// Access own "canSleep" property on "myAnotherObj"
console.log('canSleep: ', myAnotherObj.canSleep)
// Output:
// 'canSleep: ' true


// Alternative:
// Create base object
const myBaseObj = {
  isAlive: true,
  canSpeak: true,
  sayHi() {
    return 'Hello!'
  }
}

// Create new object that will inherit from "myBaseObj"
const myNewObj = {
  canWalk: true,
  canRun: true,
  __proto__: myBaseObj // set __proto__ inside an object
}

When you want to use some object as a prototype, use its name as it is, as an object. Don't try to use it, assign it in case of __proto__, as a string. That will not work.

The Object.setPrototypeOf()

The Object.setPrototypeOf() is the second option to set or change prototype of an object. When you call it, the Object.setPrototypeOf() method accepts two arguments. The first argument is the object that should be inheriting. The second argument is the object you want to inherit from.

// Create base object
const myBaseObj = {
  species: 'bird',
  isAlive: true
}

// Create new object that will inherit from "myBaseObj"
const myNewObj = {
  canFly: false,
  likesIce: true
}

// Let "myNewObj" inherit from "myBaseObj"
// by setting "myNewObj" prototype to "myBaseObj"
Object.setPrototypeOf(myNewObj, myBaseObj)

// Access inherited "species" property on "myNewObj"
console.log(myNewObj.species)
// Output:
'bird'

// Access inherited "isAlive" property on "myNewObj"
console.log(myNewObj.isAlive)
// Output:
true

// Access inherited "canFly" property on "myNewObj"
console.log(myNewObj.canFly)
// Output:
false

// Access inherited "likesIce" property on "myNewObj"
console.log(myNewObj.likesIce)
// Output:
true

The Object.getPrototypeOf()

You know how to use __proto__ and Object.setPrototypeOf() method to set a prototype of an object. When you want to get current prototype of an object you can use the Object.getPrototypeOf(). This method accept on parameter, the object whose prototype you want to get.

Before you use this method, there are some things you should know. First, it returns the prototype of given object. However, if you try to print it, or log it, it will not tell you the name of the prototype object. Instead, it will tell you what properties and methods given object inherited.

A better way to use this method is by using it to compare two objects. If the first object has the same prototype as the second, if it inherits from it, the result of this comparison will be true. Otherwise, false. This way, you can check if one object inherits from another because objects are not created equal.

// Create base object
const myBaseObj = {
  canEat: true,
  canSwim: true
}

// Create new object that will inherit from "myBaseObj"
const myNewObj = {
  canWalk: true
}

// Let "myNewObj" inherit from "myBaseObj"
// by setting "myNewObj" prototype to "myBaseObj"
Object.setPrototypeOf(myNewObj, myBaseObj)

// Test if "myNewObj" and "myBaseObj" has the same prototype
console.log(Object.getPrototypeOf(myNewObj) === myBaseObj)
// Output:
// true

// Log inherited properties of "myNewObj"
console.log(Object.getPrototypeOf(myNewObj))
// Output:
// { canEat: true, canSwim: true }

Prototypal inheritance and the value of "this"

When you use this in an object it refers to the object itself, the object in which you used it. What happens if you use this in an object and you then inherit from that object? Which object will this refer to? The answer is, the object you are currently working with, the object before the dot (myObj.someMethod()).

If you work with the base object, this will refer to that base object. If you work with an object that inherits from the base object, this will refer to that inheriting object. So, don't worry if your base object uses this in some method. It will work correctly also in case of objects inheriting from that base object.

// Create base object
const personOne = {
  name: 'Tom',
  sayHi() {
    return `Hello I am ${this.name}.`
  }
}

// Create another person that will inherit from "personOne"
const personTwo = {}

// Let "personTwo" inherit from "personOne"
Object.setPrototypeOf(personTwo, personOne)

// Change the "name" of "personTwo" to "Jack"
personTwo.name = 'Jack'

// Call the "sayHi()" method on "personTwo"
console.log(personTwo.sayHi())
// Output:
// 'Hello I am Jack.'

// Create third person that will also inherit from "personOne"
const personThree = {}

// Let "personThree" also inherit from "personOne"
Object.setPrototypeOf(personThree, personOne)

// Change the "name" of "personThree" to "Victoria"
personThree.name = 'Victoria'

// Call the "sayHi()" method on "personThree"
console.log(personThree.sayHi())
// Output:
// 'Hello I am Victoria.'

// Call the "sayHi()" method on "personOne" (the base object)
console.log(personOne.sayHi())
// Output:
// 'Hello I am Tom.'

Prototype, reading, writing and overriding

There is another question. What if one object inherits from another and you change that inheriting object? Any change you make to the inheriting object will change only to that inheriting object. The base object you are inheriting from will remain the same. This means two things.

The first one is that this prototypal inheritance relationship is read-only. You can't change the base object by changing inheriting object. You can change the base object only by changing it directly. This will also change all objects that inherit from it.

// Base object
const myObjOne = {
  name: 'Joe',
  age: 35
}

// New object
const myObjTwo = {}

// Let "myObjTwo" also inherit from "myObjOne"
Object.setPrototypeOf(myObjTwo, myObjOne)

// Change "name" property of "myObjTwo"
myObjTwo.name = 'Thomas'

// Add "email" property to "myObjTwo"
myObjTwo.email = 'thomas@paine.com'

// Log the "name" of "myObjTwo"
console.log(myObjTwo.name)
// Output:
// 'Thomas'

// Log the "email" of "myObjTwo"
console.log(myObjTwo.email)
// Output:
// 'thomas@paine.com'

// Try to log the "email" of "myObjOne"
console.log(myObjOne.email)
// Output:
// undefined

The second thing is even more interesting. You can modify inheriting objects. Not only that. You can actually override any inherited properties and methods. Since the relationship is read-only, any of these changes will influence only the inheriting object, not the base.

This means that you can have multiple objects inheriting from a single base object, and you can modify each of them. The base object will always remain unchanged.

// Base object
const personOne = {
  name: 'Joe',
  age: 35,
  sayHi() {
    return `Hi, my name is ${this.name}.`
  }
}

// Create new object
const personTwo = {}

// Let "myObjTwo" also inherit from "myObjOne"
Object.setPrototypeOf(personTwo, personOne)

// Change "name" of "personTwo"
personTwo.name = 'Kurt'

// Change/override "sayHi" method of "personTwo"
personTwo.sayHi = function() {
  return `Hallo, ich heiße ${this.name}.`
}

// Create another object
const personThree = {}

// Let "myObjTwo" also inherit from "myObjOne"
Object.setPrototypeOf(personThree, personOne)

// Change "name" of "personThree"
personThree.name = 'Louis'

// Change/override "sayHi" method of "personThree"
personThree.sayHi = function() {
  return `Salut, je m'appelle ${this.name}.`
}

console.log(personOne.sayHi())
// 'Hi, my name is Joe.'

console.log(personTwo.sayHi())
// 'Hallo, ich heiße Kurt.'

console.log(personThree.sayHi())
// "Salut, je m'appelle Louis."

Limits of prototypal inheritance

There is one last thing you should know. Every object in JavaScript can have only one prototype. This may sound like a no-brainer, but it is worth saying. You can't let one object inherit from multiple objects. The value of [[Prototype]] will be always only one object reference, or null.

If you want one object to inherit from multiple objects there is one thing you can do. You can create something like a chain. You create a base object "A" with some properties and methods. Next, you create another object "B" and let it inherit from "A". Then, you create another object "C" and let it inherit from "B".

The result of this chain will be object "C" that will be able to use anything you defined in both, object "A" and "B".

// Base object
const personOne = {
  canSee: true,
  canHear: true
}

// Create second object
const personTwo = {
  canTalk: true,
  canSing: true
}

// Create third object
const personThree = {
  canWalk: true,
  canRun: true
}

// Let "personTwo" also inherit from "personOne"
Object.setPrototypeOf(personTwo, personOne)

// Let "personThree" also inherit from "personTwo"
Object.setPrototypeOf(personThree, personTwo)

// Try to access "canSee" property on "personThree"
// The "canSee" property is inherited from "personOne"
console.log('canSee: ', personThree.canSee)
// Output:
// 'canSee: ' true

// Try to access "canTalk" property on "personThree"
// The "canTalk" property is inherited from "personTwo"
console.log('canTalk: ', personThree.canTalk)
// Output:
// 'canTalk: ' true

// Try to access "canRun" property on "personThree"
// The "canRun" property is "personThree" own property
console.log('canRun: ', personThree.canRun)
// Output:
// 'canRun: ' true

Conclusion: Objects, [[Prototype]] and prototypal inheritance in JavaScript

Prototypal inheritance is one of the topics every developer should know. It is also one of the topics that can sound more complicated than they are. I hope that this tutorial made it easier for you to understand what prototypal inheritance is, how it works and how to work with it.

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