JavaScript | Const

Difficulty Level : Basic
Last Updated : 08 Mar, 2019

ES2015 (ES6) introduced const keyword to define a new variable. The difference in const variable declaration than others is that it cannot be reassigned.

Properties:

Cannot be reassigned.
Block Scope
It can be assign on the variable on declaration line.
Primitive value.
The property of a const object can be change but it cannot be change to reference to the new object
The values inside the const array can be change, it can add new items to const arrays but it cannot reference to a new array.
Re-declaring of a const variable inside different block scope is allowed.
Cannot be Hoisted.
Create only read only reference to value.

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

<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.

<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:

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

<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.

<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 array cannot be change.

<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 object cannot be changed.

<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:

My Personal Notes

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