Javascript Variable Declaration – When to use let, var, and const (ES6)

Scope of var

var declarations are globally scoped or function/locally scoped. It is globally scoped when a var variable is declared outside a function. for example “myText” variable declare outside scope will be available globally.

   var  myText= "Hello";

    function myFunction() {
          console.log(myText);
    }

Hoisting of var

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. so you can declare a variable like below.

   function myFunction() {
          console.log(myText);
    }
   var  myText= "Hello";

Using Let

let is block scoped that means is lines of code bounded by {}.
A block lives in curly braces. Anything within curly braces is a block. So a variable declared in a block with the let is only available for use within that block.

   let myLet= "hello"; 
   if (true) {
        let myLet= "New Hello ";
        console.log(myLet);  
        //will display  " New Hello "
    }
   console.log(myLet)
//will display  "Hello "

the difference from var is let can not be redeclared in same scope this will Give ERROR . and you can not use let variable before declaration.

 let myLet= "hello 1";  
let myLet= "hello new";  
console.log(myLet)   

Using Const variable

Variables declared with the const maintain constant values. const declarations share some similarities with let declarations.

const declarations are block scoped same as let

   const  myLet= "hello"; 
   if (true) {
        const myLet= "New Hello ";
        console.log(myLet);  
        //will display  " New Hello "
    }
   console.log(myLet)
//will display  "Hello "

const cannot be updated or re-declared . Below code will give ERROR

 const myConst= "Hi";
  myConst= "Hello";
//error : Assignment to constant variable.