How does JavaScript create Objects from Function?

Tram Ho

JavaScript is a very flexible language, but at first I didn’t really like it. Perhaps by getting used to OOP and taking that thought to learn about it, I’ve always felt everything here is always messy. Functions and objects are everywhere, they blend together making me sometimes do not know what is happening. But in the end, I like it and if you look at the development process and the purpose it is towards, you can understand somewhat why JavaScript has become so.

As we all know, JavaScript is a scripting language. When manipulating the DOM, it presents itself as a Functional Programing. When the appearance of NodeJs, React, VueJs … then now, only with JavaScript, you can completely use it as an Object Oriented Programing to comfortably draw and also to be more suitable for the work. technology that we are using. This seems to make every developer using JavaScript feel comfortable and not surprised even though just starting with programming or simply switching from a certain language to.

So how can JavaScript help us so effectively? To find out, let’s find out where JavaScript used Function to create Object . By understanding this process, perhaps we will always have the answer to the above question.


You’ve probably heard a lot about it in the process of using JavaScript. If you already know what it is, you can skip this part, otherwise let’s learn about it together. Let’s take a look at the example below:

Everything looks confusing but it is really difficult to understand. It will not be difficult for you to make the opinion that a puppy seems to have inherited something from a dog to be able to run so nicely. But we have also seen that puppy actually has nothing inside it, so what is the point of this problem?

It is understandable that Object.create has created a new object inherited from an original object, but this is not necessarily inherited, because as in the above example, the puppy did not actually receive anything. Our puppy heir has only one privilege: deletgating what he wants to do for his dog . And this privilege is hidden by the puppy in a special property that every object has that __proto__ . We can also conclude that when creating an object from another object using the Object.create method, the __proto__ of the new object will always point to the original object, unless we use Object.setPrototypeOf to get it pointing. to another object.


Through the above example, we all know that __proto__ is a property that every object in JavaScript has. So what about a Function , of course it is also an object but moreover it also has another special property that has made me very confused that is the prototype . Let’s see the following example to better understand it:

As you can see, I just created a function named Dog and did nothing with it. However inside this function already has a property named prototype . As the name implies, the prototype is a place to build prototypes and then clone to other objects:

By this point, you probably already understand what happened and can confirm that every object created with the keyword new will have __proto__ pointing to the prototype of the object that created it (in the case above is the Dog function). .

Object creation

We have just passed two examples of __proto__ and prototype . If you notice you can also recognize, through those examples I have used up to 3 ways to create an object:

  • Object as literal : Use parentheses {} and inside it is a list of properties and methods of the object.
  • Object.create : Use Object.create to create a new object with __proto__ pointing to the original object.
  • Function constructor : Create an object using the new keyword. The new object will have __proto__ pointing to the prototype of the function that created it.

You might wonder why you don’t mention another type of object creation from class. Eg:

It is true that you can create an object the same way as above, but the truth is that in JavaScript there is no class concept. All you have just seen is a different type of syntax to make JavaScript more friendly for programmers who are familiar with OOP. Deeply hidden behind that shell are still Function . If you still have questions, you can verify:

Object as literal

Here’s how to create the simplest and possibly the most common object in JavaScript:

Perhaps there is no need to say much about how to create an object of this type. It can be understood as a simple key-value data type, in which the value is anything you want, but when it is a function , people will call it a method.


When you want to create multiple objects with the same properties or methods, using an object as literal is not really a good choice. You will have to repeat a lot of the same code and when you want to change something, it is worse than when you create them. At this point you may think of Object.create .

As I explained above, studentA and studentB both have __proto__ pointing to student , so when student change, they will also be updated with the corresponding changes. It can be seen that Object.create has done two things:

  • Create a new empty object
  • Point the __proto__ of the new object to the original object

To make it easier to imagine, we can also represent them through a simple code:

The results will certainly be no different.

Function constructor

Returning to the example above about Animal and change it a bit:

I’m not sure what JavaScript actually did after the new keyword, but the code below can give an overview of the whole process:

Everything works as expected and now it’s time to come back to see what we did in createObjectOf :

  • The first is to create a new empty object.
  • Retrieve the corresponding constructor and arguments from the data to be transferred.
  • Use Function.prototype.apply to call the function constructor (here Animal ) with this here as newObject .
  • Point __proto__ of newObject to the prototype of the constructor via Object.setPrototypeOf .
  • Finally, return the newly created newObject .

That’s all JavaScript has done, maybe in a different way, but the results are no different.


So we have come together to understand how an object is created in JavaScript and how they inherit each other. Hopefully the article will be useful for those who are still wondering about these issues like I’ve encountered. See you in the following articles.

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Source : Viblo