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.

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.

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.

const x; 
x = 12;

Output:

Uncaught SyntaxError: Missing initializer in const declaration

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

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.

// 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.

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'
}