In this article, we are going to learn about the scope of JavaScript. The scope manages the availability of variables or we can also say that it determines the accessibility of variables.
Types of Scopes in JavaScript:
- Block scope
- Function scope
- Local scope
- Global scope
Block scope: Earlier JavaScript had only Global Scope and Function Scope. let and const are the two new important keywords that were introduced by the ES6 and these two keywords provide Block Scope in JavaScript. ECMAScript (ES6) 2015 was the second major revision to JavaScript. Variables that are declared inside a { } block cannot be accessed from outside the block.
let keyword:
Example:
{
let x = 2;
}
x cannot be used here
var keyword:
Example:
{
var x = 2;
}
x can be used here
Variables declared with the var keyword cannot have block scope and they can be declared inside a { } block and can be accessed from outside the block.
Example:
HTML
<!DOCTYPE html><html><body> <h2 style="color:green">neveropen</h2> <script> function foo() { if (true) { var x = '1'; // Exist in function scope const y = '2'; // Exist in block scope let z = '3'; // Exist in block scope } console.log(x); console.log(y); console.log(z); } foo(); </script></body></html> |
Output (In Console):
1 y is not defined
Function scope: JavaScript has function scope and each function creates a new scope. Variables defined inside a function are not accessible from outside the function and variables declared with var, let and const are quite similar when declared inside a function.
var keyword:
Example:
function myFunction() {
var firstName = "Krishna"; // Function Scope
}
let keyword:
Example:
function myFunction() {
let firstName = "Krishna"; // Function Scope
}
const keyword:
Example:
function myFunction() {
const firstName = "Krishna"; // Function Scope
}
Local scope: Variables declared inside a function become local to the function. Local variables are created when a function starts and deleted when the function is executed. Local variables have Function Scope which means that they can only be accessed from within the function.
Example:
// This part of code cannot use firstName
function myFunction() {
let firstName = "Krishna";
// This part of code can use firstName
}
This part of code cannot use firstName
HTML
<!DOCTYPE html><html><body> <h2 style="color:green"> neveropen </h2> <script> function foo() { var x = '1'; console.log('inside function: ', x); } foo(); // Inside function: 1 console.log(x); // Error: x is not defined </script></body></html> |
Output (In Console):
inside function: 1 x is not defined
Global scope: Variables declared Globally (outside of any function) have Global Scope and Global variables can be accessed from anywhere in a program. Similar to function scope variables declared with var, let and const are quite similar when declared outside a block.
let keyword:
let x = 2; // Global scope
const keyword:
const x = 2; // Global scope
var keyword:
var x = 2; // Global scope
Example:
HTML
<!DOCTYPE html><html><body> <h2 style="color:green">neveropen</h2> <script> // Global scope var x = '1' const y = '2' let z = '3' console.log(x); // 1 console.log(y); // 2 console.log(z); // 3 function getNo() { console.log(x); // x is accessible here console.log(y); // y is accessible here console.log(z); // z is accessible here } getNo(); // 1 </script></body></html> |
Output (In Console):
1 2 3 1 2 3
