JavaScript Const

ES2015 (ES6) introduced the const keyword to define a new variable. The difference in const variable declaration to others is that it cannot be re-assigned.

Properties:

• Cannot be reassigned.
• It’s Block Scope
• It can be assigned to the variable on the declaration line.
• 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.
• Cannot be Hoisted.
• Creates only read-only references to value.

Syntax:

const const_name;
const x;

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

javascript

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

Output:

Uncaught TypeError: Assignment to constant variable.

Example 2: It describes the const variable which 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: 

90
77
45
22

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

javascript

const x;
x = 12;

Output: 

Uncaught SyntaxError: Missing initializer in const declaration

Example 4: It describes the const variable cannot be Hoisted. 

javascript

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

Output: 

Uncaught SyntaxError: Missing initializer in const declaration

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

javascript

// Changing the content of array is
// possible in cost array
const arr1 = ["pankaj", "sumit", "chandan", "ajay"];

console.log(arr1.toString());

arr1[2] = "Narayan"; // possible

console.log(arr1.toString());

Output: 

pankaj, sumit, chandan, ajay
pankaj, sumit, Narayan, ajay

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

javascript

const person = {
    first_name: "Pankaj",
    last_name: "Singh",
    Age: 20,
    About: "Web Developer and Competitive Programmer"
};

console.log(person);

// It is possible
person.first_name = "Aryan";
person.last_name = "Yadav";
person.Age = 22;
person.About = "Commerce undergraduate";

console.log(person);

// it is not possible
// const person={
// "first_name":"Aryan",
// "last_name":"Yadav",
// "Age":22,
// "About":"Commerce undergraduate"
// }

Output:

{
    first_name: 'Pankaj',
    last_name: 'Singh',
    Age: 20,
    About: 'Web Developer and Competitive Programmer'
}
{
    first_name: 'Aryan',
    last_name: 'Yadav',
    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

P.S: To clear your concept of var, const, and let please go through How to declare variables in different ways in JavaScript?