JS Scope

Global scope

var x = 1;    // Global scope  
  
let y = 2;    // Global scope  
  
const z = 3;  // Global scope

A variable declared outside a function, becomes GLOBAL.

let carName = "Volvo";  
// code here can use carName  
  
function myFunction() {  
// code here can also use carName  
}

Function scope

Variables declared within a JavaScript function are LOCAL to the function

function myFunction1() {  
  var carName = "Volvo";  // Function Scope  
}  
  
function myFunction2() {  
  let carName = "Volvo";  // Function Scope  
}  
  
function myFunction3() {  
  const carName = "Volvo";  // Function Scope  
}

Block scope

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:

{  
  let x = 2;  
}  
// x can NOT be used here

let x = 5
{
 x = 2
}
// x = 5

let x = 5
function myFunc() {
 x = 2
}
myFunc()
// x = 2

let x = 5;  
function myfunc() {  
  const x = 3;  
}  
myfunc();  
// x = 5

Variables declared with the var keyword can NOT have block scope (Not recommended).

{
 var x = 2
}
// x CAN be uded 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.

myFunction();  
  
// code here can use carName  
  
function myFunction() {  
  carName = "Volvo";  
}

Strict mode

"use strict";  
x = 3.14;       // This will cause an error because x is not declared

Global variables defines with the var keyword belong to the window object