JavaScript let is a keyword used to declare variables in JavaScript that are block scoped. Two new keywords were added in the ES6 or ES2015 version of JavaScript. Generally, it is suggested that we must use the let keyword while working with JavaScript.

Syntax:

let variable_name = value;

Block Scope: The variables which are declared inside the { } block are known as block-scoped variables. variables declared by the var keyword cannot be block-scoped.

Example: In this example, the num variable is block scoped and it cannot be accessed outside the block. If we try to access the variable outside the block it throws a reference error.

{ 
    let num=10; 
    // calling the function inside block 
    console.log(num) 
} 
// calling the function outside block throws a Error 
console.log(num)

Output:

10
Uncaught ReferenceError: num is not defined

Global Scope: A global scope variable is a variable declared in the main body of the source code, outside all functions.

Example: In this example, the num variable is a globally scoped variable and it can be accessed from anywhere in the program.

let num=10; 
console.log(num); 
function fun(){ 
    console.log(num); 
} 
fun(); // calling the function 

Output:

10 
10

Function Scope: A function scope variable is a variable declared inside a function and cannot be accessed outside the function.

Example: In this example, the num variable is declared inside the function and cannot be accessed outside the function.

function fun(){ 
    let num=10; 
    console.log(num); 
} 
fun(); //  calling the function 
console.log(num); 

Output:

10
"ReferenceError: num is not defined

Redeclaring Variables in different blocks: The variables declared using let can be redeclared inside other blocks.

Example: In this example, variable x is redeclared inside other blocks.

let x=77; 
{ 
    let x=23; 
    console.log(x); 
} 
console.log(x); 

Output:

23
77

Redeclaring Variables in the same blocks: We cannot redeclare variables using the let keyword inside the same blocks. It will throw an error.

Example: In this example, variable x is redeclared inside same blocks.

let x=77; 
    { 
        let x=23; // legal 
        console.log(x); 
    } 
    let x=67;// illegal 
    console.log(x); 

Output:

Uncaught SyntaxError: Identifier 'x' has already been declared

Does not support Hoisting: The behavior of moving the declarations on top of the script is known as hoisting.

Example: Let doe not support hoisting.

x=12; 
    console.log(x); 
    let x;  

Output:

Uncaught ReferenceError: Cannot access 'x' before initialization 

Supported Browser:

• Chrome 49 and above
• Edge 14 and above
• Firefox 44 and above
• Opera 17 and above
• Internet Explorer 11 and above
• Safari 10 and above