JavaScript Variables: A Beginner's Guide to Storing Data

Variable names must start with a letter, _, or $.
Variable names are case-sensitive (myVar is different from MyVar).
Reserved JavaScript keywords cannot be used as variable names (var, let, const).

Valid Examples:

let userName = "Alice";

let _id = 123;

let $price = 99.99;

Invalid Examples:

let 1user = "Error"; // Cannot start with a number

let let = "Error"; // Reserved keyword

Differences Between var, let, and const

Keyword	Scope	Can Reassign?	Can Redeclare?	Hoisting
var	Function Scope	Yes	Yes	Hoisted with `undefined`
let	Block Scope	Yes	No	Hoisted without initialization
const	Block Scope	No	No	Hoisted without initialization

Example Comparison:

// var example

var x = 10;

if (true) {

var x = 20; // Same variable is overwritten

}

console.log(x); // Output: 20

// let example

let y = 10;

if (true) {

let y = 20; // Different variable (block scope)

}

console.log(y); // Output: 10

// const example

const z = 10;

z = 20; // Error: Assignment to constant variable

Try it yourself

Variable Scope:

1. Global Scope:

Variables declared outside any function or block are global.
Accessible anywhere in the code.

2. Function Scope:

Variables declared with var inside a function are function-scoped.

3. Block Scope:

Variables declared with let and const inside a block {} are block-scoped.

Example

function exampleFunction() {

var functionVar = "I am function-scoped";

let blockVar = "I am block-scoped";

if (true) {

let insideBlock = "Only accessible here";

}

console.log(functionVar); // Works

// console.log(insideBlock); // Error: not defined

}

Try it yourself

Variable Hoisting:

Variables declared with var are hoisted to the top of their scope and initialized with undefined.
Variables declared with let and const are hoisted but not initialized.

Example

console.log(a); // undefined (hoisted with var)

var a = 5;

console.log(b); // Error: Cannot access 'b' before initialization

let b = 10;

Best Practices for Variables:

✅ Use const by default.
✅ Use let if the variable needs to change.
❌ Avoid var unless absolutely necessary.
✅ Use meaningful variable names (userAge, totalPrice).
✅ Follow camelCase naming convention (firstName, lastValue).

Data Types of Variables

JavaScript variables can store values of different data types:

String: "Hello"
Number: 42
Boolean: true / false
Null: null
Undefined: undefined
Object: { key: "value" }
Array: [1, 2, 3]
Function: function() {}

Example

let name = "John"; // String

let age = 30; // Number

let isActive = true; // Boolean

let user = { id: 1, name: "Alice" }; // Object

Constants in JavaScript (`const`)

Must be initialized at declaration.
Cannot be reassigned.
Object and array values can still be modified (only the reference is constant).

Example

const user = { name: "Alice" };

user.name = "Bob"; // Allowed

// user = { name: "Eve" }; // Error: Assignment to constant variable

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Declaring Variables in JavaScript:

Variable Rules:

Differences Between var, let, and const

Keyword

Scope

Can Reassign?

Can Redeclare?

Hoisting

Variable Scope:

1. Global Scope:

2. Function Scope:

3. Block Scope:

Variable Hoisting:

Best Practices for Variables:

Data Types of Variables

Constants in JavaScript (const)

Constants in JavaScript (`const`)