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

<script type="text/javascript">
    const x = 12;
    x = 13;
    x += 1;
</script>

Output:

Uncaught TypeError: Assignment to constant variable.

Example 2: It describes the const variable which contains the Block Scope. 

javascript

<script type="text/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);
</script>

Output: 

90
77
45
22

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

javascript

<script type="text/javascript">
    const x;
    x = 12;
</script>

Output: 

Uncaught SyntaxError: Missing initializer in const declaration

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

javascript

<script type="text/javascript">
    x = 3;
    console.log(x);
    const x;
</script>

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

<script type="text/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());
</script>

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

<script type="text/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"
    // }
</script>

Output:

{first_name:'Pankaj',last_name:'Singh', 
Age: 20, About:'Web Developer and Competitive Programmer'}
About
:
"Commerce undergraduate"
Age
:
22
first_name
:
"Aryan"
last_name
:
"Yadav"
[[Prototype]]
:
Object

{first_name:'Aryan',last_name:'Yadav',
Age:22, About:'Commerce undergraduate'}
About
: "Commerce undergraduate"
Age
:22
first_name
:"Aryan"
last_name
:"Yadav"
[[Prototype]]
:Object

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?