Block Scope
Before ES6 (2015), JavaScript had only Global Scope and Function Scope.

ES6 introduced two important new JavaScript keywords: let and const.

These two keywords provide Block Scope in JavaScript.

Variables declared inside a { } block cannot be accessed from outside the block:
Variables declared with the var keyword can NOT have block scope.

Variables declared inside a { } block can be accessed from outside the block.

{
  var x = 2;
}
// x CAN be used here

Variables declared with the var keyword can NOT have block scope.

Variables declared inside a { } block can be accessed from outside the block.

{
  var x = 2;
}
// x CAN be used here

Function Scope
JavaScript has function scope: Each function creates a new 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 myFunction() {
  var carName = "Volvo";   // Function Scope
}
function myFunction() {
  let carName = "Volvo";   // Function Scope
}
function myFunction() {
  const carName = "Volvo";   // Function Scope
}

Global JavaScript Variables
A variable declared outside a function, becomes GLOBAL.

let carName = "Volvo";
// code here can use carName

function myFunction() {
// code here can also use carName
}

Automatically Global
If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

It basically means if we try to define a variable without access specifiers then it is considered as global by default

myFunction();

// code here can use carName

function myFunction() {
  carName = "Volvo";
}

To avoid the above situation

• "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
• Global variables defined with the let keyword do not belong to the window object

Points to Remember->

1. Do NOT create global variables unless you intend to.
2. Your global variables (or functions) can overwrite window variables (or functions).
3. Any function, including the window object, can overwrite your global variables and functions.

The Lifetime of JavaScript Variables

1. The lifetime of a JavaScript variable starts when it is declared.
2. Function (local) variables are deleted when the function is completed.
3. In a web browser, global variables are deleted when you close the browser window (or tab).