Variables (let, const), Primitive Data Types & Dynamic Typing

Modern JavaScript provides const and let for robust, block-scoped variable declarations.

1. const vs let vs var

const (Default Choice): Block-scoped identifier that cannot be reassigned. Use this for 95% of variables.

let: Block-scoped variable that can be reassigned (e.g. loop counters, mutable states).

var (Deprecated legacy): Function-scoped with hoisting quirks. Avoid in modern ES6+ codebases!

⊞Code Example

const siteName = "Nextsem Academy";

// siteName = "Other"; // ❌ TypeError: Assignment to constant variable.

let currentScore = 100;

currentScore += 50; // ✅ Allowed: 150

2. JavaScript Primitive Data Types

String: Text data enclosed in quotes ("Full Stack", 'Nextsem', ` `).

Number: Integers and floating-point numbers (42, 3.14159).

Boolean: Logical truth values (true or false).

Undefined: Variable declared but not yet assigned a value.

Null: Intentional absence of any object value.

BigInt: Large integers beyond 2^53 - 1.

Symbol: Unique, immutable identifier token.

⊞Code Example

const developerName = "Er. Manoj Kumar";

const lessonsCompleted = 98;

const isCertified = true;

const pendingTask = null;

let feedback; // undefined

console.log(typeof developerName); // "string"

console.log(typeof lessonsCompleted); // "number"

console.log(typeof isCertified); // "boolean"

Modern JavaScript provides const and let for robust, block-scoped variable declarations.

1. const vs let vs var

const (Default Choice): Block-scoped identifier that cannot be reassigned. Use this for 95% of variables.

let: Block-scoped variable that can be reassigned (e.g. loop counters, mutable states).

var (Deprecated legacy): Function-scoped with hoisting quirks. Avoid in modern ES6+ codebases!

⊞Code Example

const siteName = "Nextsem Academy";

// siteName = "Other"; // ❌ TypeError: Assignment to constant variable.

let currentScore = 100;

currentScore += 50; // ✅ Allowed: 150

2. JavaScript Primitive Data Types

String: Text data enclosed in quotes ("Full Stack", 'Nextsem', ` `).

Number: Integers and floating-point numbers (42, 3.14159).

Boolean: Logical truth values (true or false).

Undefined: Variable declared but not yet assigned a value.

Null: Intentional absence of any object value.

BigInt: Large integers beyond 2^53 - 1.

Symbol: Unique, immutable identifier token.

⊞Code Example

const developerName = "Er. Manoj Kumar";

const lessonsCompleted = 98;

const isCertified = true;

const pendingTask = null;

let feedback; // undefined

console.log(typeof developerName); // "string"

console.log(typeof lessonsCompleted); // "number"

console.log(typeof isCertified); // "boolean"