JavaScript Scope

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:

Example

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
}

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:

Example

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".

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).