Global Scope and Local Scope Variables
1. var
Purpose: The traditional way to declare variables before ES6 (2015). Less commonly used now due to its quirks.
Scope: Function-scoped (if declared inside a function) or globally scoped (if declared outside any function).
Reassignment: Allowed.
Behavior: Variables declared with var are hoisted to the top of their scope, meaning they can be accessed even before they are declared (initialized to undefined).
// Using var
var name = "John";
name = "Doe"; // Allowed: Reassignment is fine.
console.log(x); // Undefined: x is hoisted but not initialized.
var x = 10;
if (true) {
var message = "Hi!";
}
console.log(message); // Prints: Hi! (var is not block-scoped).
2. let
Purpose: Used for variables that need to be reassigned or are block-scoped.
Scope: Block-scoped (accessible only within the block in which it's defined).
Reassignment: Allowed.
Behavior: Variables declared with let are not hoisted in the same way as var and have a "temporal dead zone" (accessing them before their declaration results in a ReferenceError).
let age = 25;
age = 26; // Allowed: Reassignment is fine.
if (true) {
let greeting = "Hello!";
console.log(greeting); // Prints: Hello!
}
console.log(greeting); // Error: greeting is not defined (block scope).
3. Const
Purpose: Used to declare variables that cannot be reassigned.
Scope: Block-scoped
Reassignment: Not allowed after the initial assignment.
Behavior: If the value is an object or array, its properties/elements can be modified, but the reference to the object cannot change.
const PI = 3.14;
PI = 3.15; // Error: Assignment to constant variable.
const obj = { name: "Alice" };
obj.name = "Bob"; // Allowed: Object properties can be changed.
obj = {}; // Error: Cannot reassign the object.
Key difference between const, let and var
