JavaScript let is a keyword used to declare variables in block scoped JavaScript. Two new keywords have been added in JavaScript version ES6 or ES2015. In general, we recommend using the let keyword when working with JavaScript.
Syntax:
let variable_name = value;
Block Scope: Variables declared inside block { } are called block scope variables whereas variables declared with the var keyword cannot be block scoped.
Example: In this example, the variable num is block scoped and it is not accessible outside the block. If we try to access the variable outside the block, it will throw a reference error.
JavaScript
{     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 Reference Error: num is not defined.
Global Scope: A global scope variable is a variable declared in the main body of the source code, outside all the functions.
Example: In this example, the marks variable is a globally scoped variable and it can be accessed from anywhere in the program.
JavaScript
let marks=50; console.log(marks); function fun(){ Â Â Â Â console.log(marks); } fun(); // calling the function |
Output:
50 50
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.
JavaScript
function fun(){ Â Â Â Â let num=10; Â Â Â Â console.log(num); } fun(); //Â calling the function console.log(num); |
Output:
10 "Reference Error: num is not defined.
Redeclaration of Variables in different blocks: The variables declared using let keyword can be redeclared inside other blocks.
Example: In below example, variable x is redeclared inside other blocks.
JavaScript
let x=77; { Â Â Â Â let x=23; Â Â Â Â console.log(x); } console.log(x); |
Output:
23 77
Redeclaration of Variables in the same blocks: We cannot redeclare variables using the let keyword inside the same blocks. It will throw an error.
Example: In the example below, variable x is redeclared inside same blocks.
JavaScript
let x=77;     {         let x=23; // legal         console.log(x);     }     let x=67;// illegal     console.log(x); |
Output:
Uncaught Syntax Error: 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 does not support hoisting.
JavaScript
x=12; Â Â Â Â console.log(x); Â Â Â Â let x;Â Â Â |
Output:
Uncaught Reference Error: 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
Note:Â To clear your concept of var, and const, and let please go through How to declare variables in different ways in JavaScript?