Introduction to JavaScript

JavaScript is a versatile, high-level programming language and one of the core technologies of the World Wide Web, alongside HTML and CSS.

JavaScript is a single-threaded language, meaning it executes one task at a time. However, through mechanisms like the Event Loop, callbacks, and asynchronous programming (such as Promises and Async/Await, it can efficiently handle multiple operations like fetching data from servers without blocking the user interface.

Data Types in JavaScript

JavaScript has eight data types that are divided into two main groups.

• Primitive Data Type
• Non Primitive Data Type

Note: NaN is not a data type. It is a special numeric value.

Primitive Data Type

Primitive values are stored directly in the memory location associated with the variable, not as references to another memory location.

Numeric Type

• Number
• BigInt

Non Numeric Type

• String
• Boolean
• Null
• Undefined
• Symbol

Number

Number data type represents both integers and floating-point values. JavaScript numbers are based on the 64-bit floating-point format (IEEE 754).

let age = 25;
let height = 5.8;

console.log(age); // 25
console.log(height); // 5.8

Safe Limit

The safe limit is the range of whole numbers that the Number data type can represent accurately (without losing precision).

• Maximum safe integer: Number.MAX_SAFE_INTEGER → 9,007,199,254,740,991
• Minimum safe integer: Number.MIN_SAFE_INTEGER → −9,007,199,254,740,991

console.log(Number.MAX_SAFE_INTEGER); // 9,007,199,254,740,991
console.log(Number.MIN_SAFE_INTEGER); // −9,007,199,254,740,991

BigInt

BigInt is a JavaScript data type used to represent very large whole numbers that are bigger than the Safe Limit.

let bigOne = 12345678901234567890n;
let bigTwo = BigInt("12345678901234567890");

console.log(bigOne); // 12345678901234567890
console.log(bigTwo); // 12345678901234567890

Note: BigInt cannot contain decimal values

String

String is used to store text in JavaScript. It can include letters, numbers, symbols, and spaces. You can create strings using single quotes, double quotes, or template literals.

let firstName = "Jagan";
let lastName = 'Ganesh';

console.log(firstName); // Jagan
console.log(lastName); // Ganesh

Template literals

Template literals are a modern way to create strings in JavaScript. They make it easier to embed variables, expressions, and multi-line text inside strings.

let name = "Jagan";
let age = 32;

let message = `My name is ${name} and I am ${age} years old.`;
console.log(message);

String literals

Characters in a string are called String Literals.

Find Length of a String

You can find the number of characters in a string using the .length property.

let city = "Chennai";

console.log(city.length); // 7

Bracket Notation in Strings

Bracket Notation lets you access individual characters of a string using their index.

// Access First Character
let name = "John";
console.log(name[0]); // "J"

// Access Last Character
let name = "Mike";
console.log(name[name.length - 1]); // "e"

String Immutability

Strings are immutable in JavaScript, which means once a string is created, you cannot change its individual characters directly. If you want a different string, you have to create a new string.

let lastName = "Doe";

// Trying to change a character does NOT work
lastName[0] = "J";
console.log(lastName); // "Doe" (remains unchanged)

// Creating a new string works
lastName = "Joe";
console.log(lastName); // "Joe"

Concatenate Strings

Strings can be concatenated using Arithmetic Operators, Compound Assignment, or by combining Variables.

Boolean

A Boolean data type has only two possible values:

• true
• false

let isLoggedIn = true;
let hasAdminAccess = false;

console.log(isLoggedIn) // true
console.log(hasAdminAccess) // false

Null

null is a data type that represents no value or empty. It is used when you want to explicitly indicate that a variable has no value.

let currentUser = null;

console.log(currentUser); // null
console.log(typeof currentUser); // object (this is a known JavaScript quirk)

Undefined

undefined is a data type that represents a variable that has been declared but has not been assigned a value. It basically means no value yet.

let name;

console.log(name);        // undefined
console.log(typeof name); // undefined

Null → "Nothing here, I put it intentionally" Think of an empty box you placed on purpose.

Undefined → "I forgot to put something here" Think of a box that exists but you never filled.

Symbol

Symbol is a unique and immutable primitive data type. Symbols are mainly used as unique identifiers for object properties, ensuring that property keys do not conflict with others.

let id1 = Symbol("id");
let id2 = Symbol("id");

console.log(id1 === id2); // false (each Symbol is unique)

Note: Symbols are perfect when you need unique, collision-free identifiers - for object keys, constants, or customizing behavior with well-known symbols.

Example

// Create unique symbols for different roles
const admin = Symbol("role");
const editor = Symbol("role");

let user = {};
user[admin] = "Alice";
user[editor] = "Bob";

console.log(user[admin]);  // Alice
console.log(user[editor]); // Bob
console.log(admin === editor); // false (each Symbol is unique)

Even though both symbols have the same description "role", they are unique identifiers.

Non-Primitive Data Type

Non-Primitive data types, also known as reference types or object types, are used to store collections of data and more complex entities.

Note: Unlike primitive types (which store the actual value), non-primitive variables store a reference to the location of the object in memory.

Objects

All non-primitive values in JavaScript are objects.

• Object
• Date
• RegExp
• Array
• Function

Object

It is used to store data in the form of key–value pairs.

let user = {
  name: "Jagan",
  age: 25,
  isActive: true
};

console.log(user.name); // Jagan
console.log(user.age);  // 25

Note: Objects are mutable, which means their values can be changed.

Array

Array values are stored in an ordered list, and each value has an index number (starting from 0).

let fruits = ["Apple", "Banana", "Orange"];

console.log(fruits[0]); // Apple
console.log(fruits[2]); // Orange

Find Length of an Array

console.log(fruits.length); // 3

Note: Arrays are mutable, meaning elements can be added, removed, or changed.

Function

A function performs a specific task and runs only when it is called.

const greet = (name) => {
  console.log("Hello " + name);
}

greet("Jagan"); // Hello Jagan

Function with Return Value

const add = (a, b) => {
    return a + b;
}

console.log(add(5, 10)); // 15

Date

The Date object is used to work with dates and times in JavaScript. It allows you to create, read, and manipulate dates such as the current date, specific dates, and time values.

let today = new Date();

console.log(today); // current date and time

Get Date Components

let now = new Date();

console.log(now.getFullYear()); // Year
console.log(now.getMonth());    // Month (0–11)
console.log(now.getDate());     // Day of the month
console.log(now.getHours());    // Hours
console.log(now.getMinutes());  // Minutes

Note: The Date object is mutable, meaning its values can be changed using setter methods like setFullYear(), setMonth(), etc.

RegExp

RegExp (Regular Expression) is used to search, match, and manipulate patterns in strings. It is especially useful for validation, searching, and text replacement.

let pattern = /hello/;
let text = "hello world";

console.log(pattern.test(text)); // true