An Introduction to JavaScript's Prototype for Beginners

Objects play a crucial role in JavaScript, serving as a fundamental element in nearly every facet of the programming language. When you began your journey in JavaScript, one of the initial topics you likely explored was the creation of objects. To delve into the intricacies of prototypes in JavaScript, it's beneficial to revisit the basics through the perspective of a junior developer.

Objects in JavaScript are essentially collections of key/value pairs. The primary method of creating an object involves using curly braces {}. Properties and methods are then added to the 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
}

Basic. In our program, it's likely that we'll have to generate multiple animals. To streamline this process, we should organize the logic into a function that we can use whenever we want to make a new animal. This approach is known as Functional Instantiation, and the function is referred to as a "constructor function" because it constructs 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, when we wish to generate a new animal or instance, we simply invoke our Animal function and provide it with the animal's name and energy level. This approach is efficient and straightforward. However, there is a notable drawback to this pattern. The issue lies with the three methods - eat, sleep, and play. These methods are not only dynamic but also entirely generic. This implies that recreating these methods for each new animal is unnecessary and results in wasted memory, making each animal object larger than necessary. Is there a solution to this problem? What if, instead of repeatedly creating these methods for every new animal, we transfer them to their own object, allowing each animal to reference that shared object? This approach can be termed as Functional Instantiation with Shared Methods, although it may sound verbose, it accurately describes the concept.

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 enhance our example further by employing Object.create. In simple terms, Object.create enables the creation of an object that delegates failed property lookups to another object. To put it another way, when a property lookup on the created object fails, Object.create allows it to check another object to see if that object contains the desired property. To make it clearer, let's take a look at 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.

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.

"WTF this dude just re-created a crappier version of a Class" - You

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)

Pretty clean, right?

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.constructor, leo doesn't have a constructor 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
}

Certainly! Here's a rewritten version:

"Why is this the case? Well, when using a for-in loop, it iterates over all enumerable properties of both the object and its prototype. Since, by default, any property added to a function's prototype is enumerable, we end up seeing not only the 'name' and 'energy' properties but also all the methods on the prototype, such as 'eat,' 'sleep,' and 'play.' To address this issue, we have two options: either specify that all prototype methods are non-enumerable, or find a way to only log to the console if the property belongs to the object itself and not to the prototype it delegates to in case of failed lookups. This is where 'hasOwnProperty' comes into play.

'hasOwnProperty' is a property present on every object that returns a boolean value, indicating whether the object possesses the specified property as its own, rather than inheriting it from the prototype. This is precisely what we need. Armed with this understanding, we can now modify our code to incorporate 'hasOwnProperty' within 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

At times, you may need to determine if an object belongs to a particular class. In such situations, the instanceof operator can be employed. While the concept is straightforward, the syntax may seem a bit unusual, especially if you are encountering it for the first time. Here's how it functions:

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.

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

In this article, we have extensively utilized Object.create to generate objects that inherit from the prototype of the constructor function. By now, you should be familiar with incorporating Object.create into your code. However, it's crucial to delve into the inner workings of Object.create. To gain a deeper understanding of its functionality, we will embark on the process of recreating Object.create. Let's start by examining what we already comprehend about the functioning of Object.create.

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 a new function, denoted as Fn in the provided code, is created, it automatically includes a prototype property. Upon invocation using the new keyword, the result is an object that will refer to the function's prototype in case of failed lookups. By altering the function's prototype, we gain control over the object to which the delegation occurs during unsuccessful lookups. In the given example, we override Fn's prototype with the object passed in during the invocation of Object.create, referred to as objToDelegateTo.

It's essential to note that in our example, support is provided for a single argument in Object.create. The official implementation also accommodates a second, optional argument that enables the addition of 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.

const Animal = () => {}
console.log(Animal.prototype) // undefined

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