You can’t get very far in JavaScript without dealing with objects. They’re foundational to almost every aspect of the JavaScript programming language. In fact, learning how to create objects is probably one of the first things you studied when you were starting out. With that said, in order to most effectively learn about prototypes in JavaScript, we’re going to channel our inner Jr. developer and go back to the basics.

Objects are key/value pairs. The most common way to create an object is with curly braces {} and you add properties and methods to an object using dot notation.

let animal = {} animal.name = 'Leo' animal.energy = 10 animal.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } animal.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } animal.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length }

Simple. Now odds are in our application we’ll need to create more than one animal. Naturally the next step for this would be to encapsulate that logic inside of a function that we can invoke whenever we needed to create a new animal. We’ll call this pattern Functional Instantiation and we'll call the function itself a "constructor function" since it's responsible for "constructing" a new object.

Functional Instantiation

function Animal (name, energy) { let animal = {} animal.name = name animal.energy = energy animal.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } animal.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } animal.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } return animal } const leo = Animal('Leo', 7) const snoop = Animal('Snoop', 10)

"I thought this was an Advanced JavaScript course...?" - Your brain It is. We'll get there.

Now whenever we want to create a new animal (or more broadly speaking a new “instance”), all we have to do is invoke our Animal function, passing it the animal's name and energy level. This works great and it's incredibly simple. However, can you spot any weaknesses with this pattern? The biggest and the one we'll attempt to solve has to do with the three methods - eat , sleep , and play . Each of those methods are not only dynamic, but they're also completely generic. What that means is that there's no reason to re-create those methods as we're currently doing whenever we create a new animal. We're just wasting memory and making each animal object bigger than it needs to be. Can you think of a solution? What if instead of re-creating those methods every time we create a new animal, we move them to their own object then we can have each animal reference that object? We can call this pattern Functional Instantiation with Shared Methods , wordy but descriptive.

Functional Instantiation with Shared Methods

const animalMethods = { eat(amount) { console.log(`${this.name} is eating.`) this.energy += amount }, sleep(length) { console.log(`${this.name} is sleeping.`) this.energy += length }, play(length) { console.log(`${this.name} is playing.`) this.energy -= length } } function Animal (name, energy) { let animal = {} animal.name = name animal.energy = energy animal.eat = animalMethods.eat animal.sleep = animalMethods.sleep animal.play = animalMethods.play return animal } const leo = Animal('Leo', 7) const snoop = Animal('Snoop', 10)

By moving the shared methods to their own object and referencing that object inside of our Animal function, we've now solved the problem of memory waste and overly large animal Objects.

Object.create

Let’s improve our example once again by using Object.create . Simply put, Object.create allows you to create an object which will delegate to another object on failed lookups. Put differently, Object.create allows you to create an object and whenever there's a failed property lookup on that object, it can consult another object to see if that other object has the property. That was a lot of words. Let's see some code.

const parent = { name: 'Stacey', age: 35, heritage: 'Irish' } const child = Object.create(parent) child.name = 'Ryan' child.age = 7 console.log(child.name) // Ryan console.log(child.age) // 7 console.log(child.heritage) // Irish

So in the example above, because child was created with Object.create(parent) , whenever there's a failed property lookup on child , JavaScript will delegate that lookup to the parent object. What that means is that even though child doesn't have a heritage property, parent does so when you log child.heritage you'll get the parent 's heritage which was Irish .

Now with Object.create in our tool shed, how can we use it in order to simplify our Animal code from earlier? Well, instead of adding all the shared methods to the animal one by one like we're doing now, we can use Object.create to delegate to the animalMethods object instead. To sound really smart, let's call this one Functional Instantiation with Shared Methods and Object.create

Functional Instantiation with Shared Methods and Object.create

const animalMethods = { eat(amount) { console.log(`${this.name} is eating.`) this.energy += amount }, sleep(length) { console.log(`${this.name} is sleeping.`) this.energy += length }, play(length) { console.log(`${this.name} is playing.`) this.energy -= length } } function Animal (name, energy) { let animal = Object.create(animalMethods) animal.name = name animal.energy = energy return animal } const leo = Animal('Leo', 7) const snoop = Animal('Snoop', 10) leo.eat(10) snoop.play(5)

So now when we call leo.eat , JavaScript will look for the eat method on the leo object. That lookup will fail, then, because of Object.create, it'll delegate to the animalMethods object which is where it'll find eat .

So far, so good. There are still some improvements we can make though. It seems just a tad “hacky” to have to manage a separate object ( animalMethods ) in order to share methods across instances. That seems like a common feature that you'd want to be implemented into the language itself. Turns out it is and it's the whole reason you're here - prototype .

Also Read:- How to Create Multiplication Table Using JavaScript

So what exactly is prototype in JavaScript? Well, simply put, every function in JavaScript has a prototype property that references an object. Anticlimactic, right? Test it out for yourself.

function doThing () {} console.log(doThing.prototype) // {}

What if instead of creating a separate object to manage our methods (like we’re doing with animalMethods ), we just put each of those methods on the Animal function's prototype? Then all we would have to do is instead of using Object.create to delegate to animalMethods , we could use it to delegate to Animal.prototype . We'll call this pattern Prototypal Instantiation .

Prototypal Instantiation

function Animal (name, energy) { let animal = Object.create(Animal.prototype) animal.name = name animal.energy = energy return animal } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } const leo = Animal('Leo', 7) const snoop = Animal('Snoop', 10) leo.eat(10) snoop.play(5)

Hopefully you just had a big “aha” moment. Again, prototype is just a property that every function in JavaScript has and, as we saw above, it allows us to share methods across all instances of a function. All our functionality is still the same but now instead of having to manage a separate object for all the methods, we can just use another object that comes built into the Animal function itself, Animal.prototype .

Let’s. Go. Deeper.

At this point we know three things:

1) How to create a constructor function.

2) How to add methods to the constructor function’s prototype.

3) How to use Object.create to delegate failed lookups to the function’s prototype.

Those three tasks seem pretty foundational to any programming language. Is JavaScript really that bad that there’s no easier, “built in” way to accomplish the same thing? As you can probably guess at this point there is, and it’s by using the new keyword.

What’s nice about the slow, methodical approach we took to get here is you’ll now have a deep understanding of exactly what the new keyword in JavaScript is doing under the hood.

Looking back at our Animal constructor, the two most important parts were creating the object and returning it. Without creating the object with Object.create , we wouldn't be able to delegate to the function's prototype on failed lookups. Without the return statement, we wouldn't ever get back the created object.

function Animal (name, energy) { let animal = Object.create(Animal.prototype) animal.name = name animal.energy = energy return animal }

Here’s the cool thing about new - when you invoke a function using the new keyword, those two lines are done for you implicitly ("under the hood") and the object that is created is called this .

Using comments to show what happens under the hood and assuming the Animal constructor is called with the new keyword, it can be re-written as this.

function Animal (name, energy) { // const this = Object.create(Animal.prototype) this.name = name this.energy = energy // return this } const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10) and without the “under the hood” comments function Animal (name, energy) { this.name = name this.energy = energy } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10)

Again the reason this works and that the this object is created for us is because we called the constructor function with the new keyword. If you leave off new when you invoke the function, that this object never gets created nor does it get implicitly returned. We can see the issue with this in the example below.

function Animal (name, energy) { this.name = name this.energy = energy } const leo = Animal('Leo', 7) console.log(leo) // undefined

The name for this pattern is Pseudoclassical Instantiation .

If JavaScript isn’t your first programming language, you might be getting a little restless.

For those unfamiliar, a Class allows you to create a blueprint for an object. Then whenever you create an instance of that Class, you get an object with the properties and methods defined in the blueprint.

Sound familiar? That’s basically what we did with our Animal constructor function above. However, instead of using the class keyword, we just used a regular old JavaScript function to re-create the same functionality. Granted, it took a little extra work as well as some knowledge about what happens "under the hood" of JavaScript but the results are the same.

Here’s the good news. JavaScript isn’t a dead language. It’s constantly being improved and added to by the TC-39 committee. What that means is that even though the initial version of JavaScript didn’t support classes, there’s no reason they can’t be added to the official specification. In fact, that’s exactly what the TC-39 committee did. In 2015, EcmaScript (the official JavaScript specification) 6 was released with support for Classes and the class keyword. Let's see how our Animal constructor function above would look like with the new class syntax.

class Animal { constructor(name, energy) { this.name = name this.energy = energy } eat(amount) { console.log(`${this.name} is eating.`) this.energy += amount } sleep(length) { console.log(`${this.name} is sleeping.`) this.energy += length } play(length) { console.log(`${this.name} is playing.`) this.energy -= length } } const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10)

So if this is the new way to create classes, why did we spend so much time going over the old way? The reason for that is because the new way (with the class keyword) is primarily just "syntactical sugar" over the existing way we've called the pseudoclassical pattern. In order to fully understand the convenience syntax of ES6 classes, you first must understand the pseudoclassical pattern.

At this point we’ve covered the fundamentals of JavaScript’s prototype. The rest of this post will be dedicated to understanding other “good to know” topics related to it. In another post we’ll look at how we can take these fundamentals and use them to understand how inheritance works in JavaScript.

Array Methods

We talked in depth above about how if you want to share methods across instances of a class, you should stick those methods on the class’ (or function’s) prototype. We can see this same pattern demonstrated if we look at the Array class. Historically you've probably created your arrays like this

const friends = []

Turns out that’s just sugar over creating a new instance of the Array class.

const friendsWithSugar = [] const friendsWithoutSugar = new Array()

One thing you might have never thought about is how does every instance of an array have all of those built in methods ( splice , slice , pop , etc)?

Well as you now know, it’s because those methods live on Array.prototype and when you create a new instance of Array , you use the new keyword which sets up that delegation to Array.prototype on failed lookups.

We can see all the array’s methods by simply logging Array.prototype .

console.log(Array.prototype) /* concat: ƒn concat() constructor: ƒn Array() copyWithin: ƒn copyWithin() entries: ƒn entries() every: ƒn every() fill: ƒn fill() filter: ƒn filter() find: ƒn find() findIndex: ƒn findIndex() forEach: ƒn forEach() includes: ƒn includes() indexOf: ƒn indexOf() join: ƒn join() keys: ƒn keys() lastIndexOf: ƒn lastIndexOf() length: 0n map: ƒn map() pop: ƒn pop() push: ƒn push() reduce: ƒn reduce() reduceRight: ƒn reduceRight() reverse: ƒn reverse() shift: ƒn shift() slice: ƒn slice() some: ƒn some() sort: ƒn sort() splice: ƒn splice() toLocaleString: ƒn toLocaleString() toString: ƒn toString() unshift: ƒn unshift() values: ƒn values() */

The exact same logic exists for Objects as well. Alls object will delegate to Object.prototype on failed lookups which is why all Objects have methods like toString and hasOwnProperty .

Static Methods

Up until this point we’ve covered the why and how of sharing methods between instances of a Class. However, what if we had a method that was important to the Class, but didn’t need to be shared across instances? For example, what if we had a function that took in an array of Animal instances and determined which one needed to be fed next? We'll call it nextToEat .

function nextToEat (animals) { const sortedByLeastEnergy = animals.sort((a,b) => { return a.energy - b.energy }) return sortedByLeastEnergy[0].name }

It doesn’t make sense to have nextToEat live on Animal.prototype since we don't want to share it amongst all instances. Instead, we can think of it as more of a helper method. So if nextToEat shouldn't live on Animal.prototype , where should we put it? Well the obvious answer is we could just stick nextToEat in the same scope as our Animal class then reference it when we need it as we normally would.

class Animal { constructor(name, energy) { this.name = name this.energy = energy } eat(amount) { console.log(`${this.name} is eating.`) this.energy += amount } sleep(length) { console.log(`${this.name} is sleeping.`) this.energy += length } play(length) { console.log(`${this.name} is playing.`) this.energy -= length } } function nextToEat (animals) { const sortedByLeastEnergy = animals.sort((a,b) => { return a.energy - b.energy }) return sortedByLeastEnergy[0].name } const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10) console.log(nextToEat([leo, snoop])) // Leo

Now this works, but there’s a better way.

Whenever you have a method that is specific to a class itself, but doesn’t need to be shared across instances of that class, you can add it as a static property of the class.

class Animal { constructor(name, energy) { this.name = name this.energy = energy } eat(amount) { console.log(`${this.name} is eating.`) this.energy += amount } sleep(length) { console.log(`${this.name} is sleeping.`) this.energy += length } play(length) { console.log(`${this.name} is playing.`) this.energy -= length } static nextToEat(animals) { const sortedByLeastEnergy = animals.sort((a,b) => { return a.energy - b.energy }) return sortedByLeastEnergy[0].name } }

Now, because we added nextToEat as a static property on the class, it lives on the Animal class itself (not its prototype) and can be accessed using Animal.nextToEat .

const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10) console.log(Animal.nextToEat([leo, snoop])) // Leo

Because we’ve followed a similar pattern throughout this post, let’s take a look at how we would accomplish this same thing using ES5. In the example above we saw how using the static keyword would put the method directly onto the class itself. With ES5, this same pattern is as simple as just manually adding the method to the function object.

function Animal (name, energy) { this.name = name this.energy = energy } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } Animal.nextToEat = function (nextToEat) { const sortedByLeastEnergy = animals.sort((a,b) => { return a.energy - b.energy }) return sortedByLeastEnergy[0].name } const leo = new Animal('Leo', 7) const snoop = new Animal('Snoop', 10) console.log(Animal.nextToEat([leo, snoop])) // Leo

Getting the prototype of an object

Regardless of whichever pattern you used to create an object, getting that object’s prototype can be accomplished using the Object.getPrototypeOf method.

function Animal (name, energy) { this.name = name this.energy = energy } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } const leo = new Animal('Leo', 7) const prototype = Object.getPrototypeOf(leo) console.log(prototype) // {constructor: ƒ, eat: ƒ, sleep: ƒ, play: ƒ} prototype === Animal.prototype // true

There are two important takeaways from the code above.

First, you’ll notice that proto is an object with 4 methods, constructor , eat , sleep , and play . That makes sense. We used getPrototypeOf passing in the instance, leo getting back that instances' prototype, which is where all of our methods are living. This tells us one more thing about prototype as well that we haven't talked about yet. By default, the prototype object will have a constructor property which points to the original function or the class that the instance was created from. What this also means is that because JavaScript puts a constructor property on the prototype by default, any instances will be able to access their constructor via instance.constructor .

The second important takeaway from above is that Object.getPrototypeOf(leo) === Animal.prototype . That makes sense as well. The Animal constructor function has a prototype property where we can share methods across all instances and getPrototypeOf allows us to see the prototype of the instance itself.

function Animal (name, energy) { this.name = name this.energy = energy } const leo = new Animal('Leo', 7) console.log(leo.constructor) // Logs the constructor function

To tie in what we talked about earlier with Object.create , the reason this works is because any instances of Animal are going to delegate to Animal.prototype on failed lookups. So when you try to access leo.prototype , leo doesn't have a prototype property so it will delegate that lookup to Animal.prototype which indeed does have a constructor property. If this paragraph didn't make sense, go back and read about Object.create above.

You may have seen __proto__ used before to get an instances’ prototype. That’s a relic of the past. Instead, use Object.getPrototypeOf(instance) as we saw above.

Determining if a property lives on the prototype

There are certain cases where you need to know if a property lives on the instance itself or if it lives on the prototype the object delegates to. We can see this in action by looping over our leo object we've been creating. Let's say the goal was the loop over leo and log all of its keys and values. Using a for in loop, that would probably look like this.

function Animal (name, energy) { this.name = name this.energy = energy } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } const leo = new Animal('Leo', 7) for(let key in leo) { console.log(`Key: ${key}. Value: ${leo[key]}`) } What would you expect to see? Most likely, it was something like this - Key: name. Value: Leo Key: energy. Value: 7 However, what you saw if you ran the code was this - Key: name. Value: Leo Key: energy. Value: 7 Key: eat. Value: function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Key: sleep. Value: function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Key: play. Value: function (length) { console.log(`${this.name} is playing.`) this.energy -= length }

Why is that? Well a for in loop is going to loop over all of the enumerable properties on both the object itself as well as the prototype it delegates to. Because by default any property you add to the function's prototype is enumerable, we see not only name and energy , but we also see all the methods on the prototype - eat , sleep , and play . To fix this, we either need to specify that all of the prototype methods are non-enumerable or we need a way to only console.log if the property is on the leo object itself and not the prototype that leo delegates to on failed lookups. This is where hasOwnProperty can help us out.

hasOwnProperty is a property on every object that returns a boolean indicating whether the object has the specified property as its own property rather than on the prototype the object delegates to. That's exactly what we need. Now with this new knowledge we can modify our code to take advantage of hasOwnProperty inside of our for in loop.

... const leo = new Animal('Leo', 7) for(let key in leo) { if (leo.hasOwnProperty(key)) { console.log(`Key: ${key}. Value: ${leo[key]}`) } }

And now what we see are only the properties that are on the leo object itself rather than on the prototype leo delegates to as well.

Key: name. Value: Leo Key: energy. Value: 7

If you’re still a tad confused about hasOwnProperty , here is some code that may clear it up.

function Animal (name, energy) { this.name = name this.energy = energy } Animal.prototype.eat = function (amount) { console.log(`${this.name} is eating.`) this.energy += amount } Animal.prototype.sleep = function (length) { console.log(`${this.name} is sleeping.`) this.energy += length } Animal.prototype.play = function (length) { console.log(`${this.name} is playing.`) this.energy -= length } const leo = new Animal('Leo', 7) leo.hasOwnProperty('name') // true leo.hasOwnProperty('energy') // true leo.hasOwnProperty('eat') // false leo.hasOwnProperty('sleep') // false leo.hasOwnProperty('play') // false

Check if an object is an instance of a Class

Sometimes you want to know whether an object is an instance of a specific class. To do this, you can use the instanceof operator. The use case is pretty straight forward but the actual syntax is a bit weird if you've never seen it before. It works like this

object instanceof Class

The statement above will return true if object is an instance of Class and false if it isn't. Going back to our Animal example we'd have something like this.

function Animal (name, energy) { this.name = name this.energy = energy } function User () {} const leo = new Animal('Leo', 7) leo instanceof Animal // true leo instanceof User // false

The way that instanceof works is it checks for the presence of constructor.prototype in the object's prototype chain. In the example above, leo instanceof Animal is true because Object.getPrototypeOf(leo) === Animal.prototype . In addition, leo instanceof User is false because Object.getPrototypeOf(leo) !== User.prototype .

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Creating new agnostic constructor functions

Can you spot the error in the code below?

function Animal (name, energy) { this.name = name this.energy = energy }

const leo = Animal('Leo', 7)

Even seasoned JavaScript developers will sometimes get tripped up on the example above. Because we’re using the pseudoclassical pattern that we learned about earlier, when the Animal constructor function is invoked, we need to make sure we invoke it with the new keyword. If we don't, then the this keyword won't be created and it also won't be implicitly returned.

As a refresher, the commented out lines are what happens behind the scenes when you use the new keyword on a function.

function Animal (name, energy) { // const this = Object.create(Animal.prototype) this.name = name this.energy = energy // return this }

This seems like too important of a detail to leave up to other developers to remember. Assuming we’re working on a team with other developers, is there a way we could ensure that our Animal constructor is always invoked with the new keyword? Turns out there is and it's by using the instanceof operator we learned about previously.

If the constructor was called with the new keyword, then this inside of the body of the constructor will be an instanceof the constructor function itself. That was a lot of big words. Here's some code.

function Animal (name, energy) { if (this instanceof Animal === false) { console.warn('Forgot to call Animal with the new keyword') } this.name = name this.energy = energy }

Now instead of just logging a warning to the consumer of the function, what if we re-invoke the function, but with the new keyword this time?

function Animal (name, energy) { if (this instanceof Animal === false) { return new Animal(name, energy) } this.name = name this.energy = energy }

Now regardless of if Animal is invoked with the new keyword, it'll still work properly.

Re-creating Object.create

Throughout this post we’ve relied heavily upon Object.create in order to create objects which delegate to the constructor function's prototype. At this point, you should know how to use Object.create inside of your code but one thing that you might not have thought of is how Object.create actually works under the hood. In order for you to really understand how Object.create works, we're going to re-create it ourselves. First, what do we know about how Object.create works?

1) It takes in an argument that is an object.

2) It creates an object that delegates to the argument object on failed lookups.

3) It returns the new created object.

Let’s start off with #1.

Object.create = function (objToDelegateTo) { }

Simple enough.

Now #2?—?we need to create an object that will delegate to the argument object on failed lookups. This one is a little more tricky. To do this, we’ll use our knowledge of how the new keyword and prototypes work in JavaScript. First, inside the body of our Object.create implementation, we'll create an empty function. Then, we'll set the prototype of that empty function equal to the argument object. Then, in order to create a new object, we'll invoke our empty function using the new keyword. If we return that newly created object, that'll finish #3 as well.

Object.create = function (objToDelegateTo) { function Fn(){} Fn.prototype = objToDelegateTo return new Fn() }

Wild. Let’s walk through it.

When we create a new function, Fn in the code above, it comes with a prototype property. When we invoke it with the new keyword, we know what we'll get back is an object that will delegate to the function's prototype on failed lookups. If we override the function's prototype, then we can decide which object to delegate to on failed lookups. So in our example above, we override Fn 's prototype with the object that was passed in when Object.create was invoked which we call objToDelegateTo .

Note that we’re only supporting a single argument to Object.create. The official implementation also supports a second, optional argument which allow you to add more properties to the created object.

Arrow Functions

Arrow functions don’t have their own this keyword. As a result, arrow functions can't be constructor functions and if you try to invoke an arrow function with the new keyword, it'll throw an error.

const Animal = () => {}

const leo = new Animal() // Error: Animal is not a constructor

Also, because we demonstrated above that the pseudoclassical pattern can’t be used with arrow functions, arrow functions also don’t have a prototype property.