What is the difference between primitives types and objects in JavaScript?

In this short tutorial we are going to look at the differences between primitive types and objects in JavaScript. To start with, we're going to look at what primitive types are. Then we will see what objects are and how they differ. 

Introduction

The primitive types in JavaScript are as follows:

• Boolean: 'boolean
• String: 'string
• Number: 'number', 'bigint', 'bigint'.
• Symbol: 'symbol
• Undefined: 'undefined

As a general rule, anything that is not a primitive type in JavaScript is an object of type 'object'.

You are probably wondering why the type null is not in the list. This is because null is an object type.

To get the type of a variable in JavaScript you can use the typeof operator. In fact, if you run the expression typeof null you will see that the result you get is 'object'. This is one of the big differences between null and undefined, as we have seen in the tutorial where we explained the differences between null and undefined.

As for functions, it is true that they are of type function, although we have not included this type in the list of primitive types. This is because the constructor of the function type is derived from the object type.

Differences between primitive types and objects

We will now look at the most notable differences between primitive types and objects:

• Primitive types are always passed by value, while objects are passed by reference.
• Primitive types are copied by value while objects are copied by reference.
• Primitive types are compared by value while objects are compared by reference.
• Primitive types are immutable, while the only immutable element of an object is its reference, and its value can be modified.

We will now look at some examples that validate these assertions.

To begin with, let's copy an object of primitive type to see what happens:

let animal = 'cat';  
let pet = animal;

Now let's change the value of the animal variable and see what happens to the pet variable:

animal = 'dinosaur';  
console.log(pet);

The value that will be shown on the screen will be 'cat', because when working with primitive types, the value assignment that we have done at the beginning has been done by value and not by reference. That is, although we can assign the value of one variable to another, they are totally independent.

Let's see now what happens when we copy an object:

let animal = {    
      species: 'cat'  
}    
let pet = animal;

In this example, the pet variable points to the same object as the animal variable, since the assignment has been made by reference and not by value.

To demonstrate this, let's modify the species property:

animal.species= 'dinosaur'; 
console.log(pet.species);

The value that will be displayed on the screen will be 'dinosaur', since pet pointed to the same object as animal. In other words, we actually have a single object with two references to it. In fact, if we compare the animal object with the pet object we will see that the variables are identical:

if (
animal === pet) {    
console.log('We are the same object');  
}

However, if we define two different objects, the result of the comparison will be false even if the objects are identical, as they will have a different reference:

let animal = {    
species: 'cat'  
}    
let pet = {    
species: 'dinosaurio'  
}    
if (animal !==  pet) {    
console.log('We are not the same object');  
}