JavaScript Scope
Scope determines the accessibility (visibility) of variables.
JavaScript variables have 3 types of scope:
- Global scope
- Function scope
- Block scope
Global Scope
Variables declared Globally (outside any block or function) have Global Scope.
Global variables can be accessed from anywhere in a JavaScript program.
Variables declared with var, let
and const are
quite similar when declared outside a block.
They all have Global Scope:
var x = 1; // Global scope
let y = 2; // Global scope
const z = 3; // Global scope
Example
A variable declared outside a function, becomes GLOBAL.
let carName = "Volvo";
// code here can use carName
function myFunction() {
// code here can also use carName
}
Try it Yourself »
A global variable has Global Scope:
All scripts and functions in the same web page can access a variable with global scope.
Function Scope
Each JavaScript function have their own scope.
Variables defined inside a function are not accessible (visible) from outside the function.
Variables declared with var, let
and const are
quite similar when declared inside a function.
They all have Function Scope:
function myFunction1() {
var carName = "Volvo"; // Function Scope
}
function myFunction2() {
let carName = "Volvo"; // Function Scope
}
function myFunction3() {
const carName = "Volvo"; // Function Scope
}
A function variable has Function Scope:
No scripts or functions outside the function can access it.
Local Scope
Variables declared within a JavaScript function, are LOCAL to the function:
Example
// code here can NOT use carName
function myFunction() {
let carName = "Volvo";
// code here CAN use carName
}
// code here can NOT use carName
Try it Yourself »
Local variables have Function Scope:
They can only be accessed from within the function.
Variables with the same name can be used in different functions.
Block Scope
Before ES6, JavaScript variables could only have Global Scope or Function Scope.
ES6 introduced two important new JavaScript keywords: let and const.
These two keywords provide Block Scope in JavaScript.
Variables declared with let and const
inside a code block are "block-scoped," meaning they are only accessible within that block.
This helps prevent unintended variable overwrites and promotes better code organization:
Example
{
let x = 2;
}
// x can NOT be used here
Variables declared with the var keyword can NOT have block scope.
Variables declared with the var keyword,
inside a { } block; can be accessed from outside the block.
Example
{
var x = 2;
}
// x CAN be used here
The Lifetime of JavaScript Variables
The lifetime of a JavaScript variable starts when it is declared.
Function (local) variables are created when a function starts, and deleted when the function is completed.
In a web browser, global variables are deleted when you close the browser window (or tab).
