JavaScript variables

JavaScript Variables can be declared in 4 ways:

• Automatically
• var
• let
• const

1. Automatically

They are automatically declared when first used:

x = 5;
y = 6;
z = x + y; // Output: 11

2.var

• Scope: Function-scoped or globally scoped if declared outside any function.
• Re-declaration: Allows re-declaration within the same scope, which can lead to unexpected behavior.
• Hoisting: Variables declared with var are hoisted to the top of their scope (but only their declaration, not their initialization).

Example:

var name = "Alice";
var name = "Bob"; // No error, allowed in the same scope

3. let

• Scope: Block-scoped, meaning it’s only accessible within the { } it was defined in.
• Re-declaration: Cannot be re-declared within the same scope, helping to avoid errors.
• Hoisting: let variables are also hoisted, but unlike var, they are not initialized. They enter a “temporal dead zone” from the start of the block until their declaration, which prevents access before they are declared.

Example:

let age = 25;
let age = 30; // Error: can't re-declare in the same scope

3. const

• Scope: Block-scoped, similar to let.
• Re-declaration: Cannot be re-declared or reassigned within the same scope.
• Initialization: Must be initialized at the time of declaration, as it cannot be assigned later.
• Immutability: The variable itself is immutable (i.e., it can’t be reassigned), but if it’s an object or array, the properties or elements inside can be modified.

Example:

const city = "New York";
city = "Los Angeles"; // Error: cannot reassign a `const` variable

Summary

Keyword	Scope	Re-declaration	Reassignment	Hoisting
var	Function / Global	Allowed	Allowed	Yes (initially undefined)
let	Block	Not allowed	Allowed	Yes (in “temporal dead zone”)
const	Block	Not allowed	Not allowed	Yes (in “temporal dead zone”)