JavaScript Const (2024) -

ES2015 (ES6) introduced the const keyword to define a new variable. The basic difference between const variable declaration and other variables are that it cannot be reassigned.

Properties:

Const cannot be reassigned.
It’s Block Scope
It can be assigned to the variable on the declaration line itself.
It’s a Primitive value.
The property of a const object can be changed but it cannot be changed to a reference to the new object.
The values inside the const array can be changed, it can add new items to const arrays but it cannot reference a new array.
Re-declaring of a const variable inside different block scopes is allowed.
Const cannot be Hoisted.
Const creates only read-only references to value.

Syntax:

const const_name;
const x;

Example 1: It describes that the const variable cannot be reassigned again.

JavaScript

const x = 12; 
x = 13; 
x += 1; 

Output:

Uncaught Type Error: Assignment to constant variable.

Example 2: Following example describes that const variables contains the Block Scope.

JavaScript

const x = 22; 
{ 
    const x = 90; 
    console.log(x); 
 
    { 
        const x = 77; 
        console.log(x); 
    } 
    { 
        const x = 45; 
        console.log(x); 
    } 
} 
console.log(x); 

Output:

Example 3: It describes the const variable and assigned value to it after declaration.

JavaScript

const x; 
x = 12; 

Output:

Uncaught SyntaxError: Missing initializer in const declaration

Example 4: Following example shows that const variable cannot be Hoisted.

JavaScript

x = 3; 
console.log(x); 
const x; 

Output:

Uncaught SyntaxError: Missing initializer in const declaration

Example 5: Following example describes that the array values can be modified only but reference to the array cannot be changed.

JavaScript

// Changing the content of array is 
// possible in cost array 
const arr1 = ["a", "b", "c", "d"]; 
 
console.log(arr1.toString()); 
 
arr1[2] = "z"; // possible 
 
console.log(arr1.toString()); 

Output:

a, b, c, d
a, b, z, d

Example 6: It describes that the object properties can be modified only but reference to the object cannot be changed.

JavaScript

const person = { 
    first_name: "Rohit", 
    last_name: "Singh", 
    Age: 20, 
    About: "Web Developer and Programmer"
}; 
 
console.log(person); 
 
// It is possible 
person.first_name = "Aryan"; 
person.last_name = "kumar"; 
person.Age = 22; 
person.About = "Commerce undergraduate"; 
 
console.log(person); 
 
// it is not possible 
// const person={ 
// "first_name":"Aryan", 
// "last_name":"kumar", 
// "Age":22, 
// "About":"Commerce undergraduate" 
// } 

Output:

{
    first_name: 'Rohit',
    last_name: 'Singh',
    Age: 20,
    About: 'Web Developer and Programmer'
}
{
    first_name: 'Aryan',
    last_name: 'kumar',
    Age: 22,
    About: 'Commerce undergraduate'
}

Supported Browsers:

chrome 21 and above
Edge 12 and above
Firefox 36 and above
Internet Explorer 11 and above
Opera 9 and above
Safari 5.1 and above

Note: To clear your concept of var, const, and let you need to go through how to declare variables in different ways in JavaScript? We will post its complete article soon.

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