JavaScript - The 'const' keyword

Introduction

Namaste, In this blog I will discuss the below topics.

1. What is the 'const' keyword?

2. Using the 'const' keyword

3. Complete source code

What is the 'const' keyword?

The variables declared using 'const' keyword cannot be changed i.e. once declared they are not updatable.

Using the 'const' keyword

Below are various scenarios of using the 'const' keyword

1. The variable cannot be updated after the declaration

   
    const myval=10;
    console.log("type", typeof myval, " value", myval);
    // result : type number  value 10
    //myval = "ram"; //error
   

2. Cannot be re-declared

    const myval = "Hari";
    //const myval =12; // error - cannot be re-declared
   

3. Has block scope i.e. it cannot be accessed outside a code block

    // Has block scope
    if (myval > 6) {
        const myval2=11;
    }
    //console.log("type", typeof myval2, " value", myval2); // error
   
    //block scope in function
    function fx() {
        if (myval > 6) {
            const myval4=8;        
        }
        //console.log("myval 4", myval4); //error
    }
    fx();
   

4. It is not hoisted i.e. it can only be accessed after initialization

    // not hoisted
    //console.log("myval3", myval3); // error
    const myval3=12;
   

5. The object declared is also constant but the elements inside it are updatable

    const obj = {
        no:10,
        name: "Ram"
    };
    //obj = 100; // error
    obj.no = 200; // no-error , works
   

Complete Source Code

//Objective: Using the const keyword
//Author: Mahavir DS Rathore

const myval=10;
console.log("type", typeof myval, " value", myval);
// result : type number  value 10
//myval = "ram"; //error

//const myval =12; // error - cannot be re-declared 

// Has block scope
if (myval > 6) {
    const myval2=11;
}
//console.log("type", typeof myval2, " value", myval2); // error

// not hoisted
//console.log("myval3", myval3); // error
const myval3=12;

//block scope in function
function fx() {
    if (myval > 6) {
        const myval4=8;        
    }
    //console.log("myval 4", myval4); //error
}
fx();

const obj = {
    no:10,
    name: "Ram"
};

//obj = 100; // error
obj.no = 200; // no-error , works