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.
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 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.
javascript
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.
javascript
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.
javascript
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.
javascript
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.
javascript
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
P.S: To clear your concept of var, and const, and let please go through How to declare variables in different ways in JavaScript?
