Scope = Visibility
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
let carName = "Volvo";
// code here can use carName
function myFunction() {
// code here can also use carName
}
Note: Global variables has Global Scope: All scripts and functions in the same web page can access a variable with global scope.
Function Scope
All JavaScript functions 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
}
Example
// code here can NOT use carName
function myFunction() {
let carName = "Volvo";
// code here CAN use carName
}
// code here can NOT use carName
Local Variables have Function Scope
- They can only be accessed from within the function
- No scripts or functions outside the function can access them
- Variables with the same name can be used outside the function
- Variables with the same name can be used in different functions
- Local variables are created when a function starts
- Local variables are deleted when the function is completed
- Arguments (parameters) work as local variables inside 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 (Not Recommended)
{
var x = 2;
}
// x CAN be used here
Automatically Global
If you assign a value to a variable that has not been declared, it will become a GLOBAL variable.
This code example will declare a global variable carName, even if the value is assigned inside a function.
Example
myFunction();
// code here can use carName
function myFunction() {
carName = "Volvo";
}
Strict Mode
All modern browsers support running JavaScript in "Strict Mode".
Warning: In "Strict Mode", undeclared variables are not automatically global.
Global Variables in HTML
With JavaScript, the global scope is the JavaScript environment.
In HTML, the global scope is the window object.
Global variables defined with the var keyword belong to the window object:
Example (Not Recommended)
var carName = "Volvo";
// code here
can use window.carName
Global variables defined with the let keyword do not belong to the window object:
Example
let carName = "Volvo";
// code here can NOT use window.carName
Warning: Do NOT create global variables unless you intend to. Your global variables (or functions) can overwrite window variables (or functions). Any function, including the window object, can overwrite your global variables and functions.
The Lifetime of JavaScript Variables
The lifetime of a JavaScript variable starts when it is declared.
Function (local) variables are deleted when the function is completed.
In a web browser, global variables are deleted when you close the browser window (or tab).
