Module 2: JavaScript Basics

Summary

• Variables (let): Re-declarable, re-assignable.

• Constants (const): Cannot be re-declared or re-assigned.

• Comments:

  • Single-line: // This is a comment

  • Multi-line: /* This is a multi-line comment */

Example

let age = 25; // Variable (mutable)

const PI = 3.14159; // Constant (immutable)

// This function calculates area

function calculateArea() {

/* Multi-line comment

explaining the function

*/

}

Summary

• camelCase: userName (variables, functions).

• PascalCase: UserProfile (classes, constructors).

• snake_case: user_name (rare in JS, used in URLs).

• UPPER_SNAKE_CASE: MAX_USERS (constants).

Example

let userName = "Alice"; // camelCase

class UserProfile {} // PascalCase

const MAX_USERS = 100; // UPPER_SNAKE_CASE

Summary

JavaScript has primitive and reference types:

Example

let name = "Bob"; // string

let score = 10; // number

let isLoggedIn = false; // boolean

let user = null; // null

let data; // undefined

let person = { age: 30 }; // object

Summary

Strings are immutable (cannot be changed after creation). Use methods to manipulate them.

Common Methods

• length: Gets string length.

• toUpperCase()/toLowerCase(): Changes case.

• slice(start, end): Extracts part of a string.

• replace(old, new): Replaces substring.

• includes(search): Checks if substring exists.

Example

let message = "Hello, World!";

console.log(message.length); // 13

console.log(message.toUpperCase()); // "HELLO, WORLD!"

console.log(message.slice(7, 12)); // "World"

Summary

• Integers, floats, and NaN (Not-a-Number).

• Use Math object for operations (e.g., Math.random()).

Example

let sum = 10 + 5; // 15

let price = 9.99; // float

let random = Math.random(); // 0 ≤ random < 1

Summary

Use backticks (`) for multi-line strings and embed expressions.

Example

let name = "Alice";

let age = 25;

console.log(`Hello, ${name}! You are ${age} years old.`);

// Output: "Hello, Alice! You are 25 years old."

Summary

Ordered lists of values. Use methods to add/remove items.

Common Methods

• push(): Adds to end.

• pop(): Removes from end.

• slice(): Extracts part of array.

Example

let fruits = ["apple", "banana", "cherry"];

fruits.push("date"); // ["apple", "banana", "cherry", "date"]

let last = fruits.pop(); // "date" (array becomes ["apple", "banana", "cherry"])

Summary

• null: Assigned to indicate "no value".

• undefined: Value not assigned or function returns no value.

Example

let data = null; // Intentional absence of value

let result; // undefined (not initialized)

function noReturn() {}

console.log(noReturn()); // undefined

Summary

• Comparisons:

  • == (loose), === (strict).

  • >, <, >=, <=, ! (not).

Example

let a = 5;

let b = "5";

console.log(a == b); // true (loose comparison)

console.log(a === b); // false (strict comparison: different types)

Summary

• Loose (==): Converts types before comparing.

• Strict (===): No type conversion.

Example

console.log(0 == false); // true (0 == 0 → true)

console.log(0 === false); // false (different types)

Summary

• Implicit: JS converts types automatically (e.g., "5" + 2 = "52").

• Explicit: Use Number(), String(), Boolean().

Example

console.log("5" + 2); // "52" (implicit string concatenation)

console.log(Number("5") + 2); // 7 (explicit conversion)

Q1: What’s the difference between let and const?

A: let allows reassignment; const does not.

Q2: Why is null different from undefined?

A: null is an intentional absence of value; undefined means uninitialized.

Q3: When should you use === over ==?

A: Always use === to avoid unexpected type coercion.

Q4: What does NaN stand for?

A: Not-a-Number. It results from invalid numeric operations (e.g., 0/0).

Q5: How do template literals improve string handling?

A: They allow embedded expressions (\${}) and multi-line strings without concatenation.

const PI = 3.14159;

let favoriteFoods = ["pizza", "sushi", "tacos"];

console.log(`I love ${favoriteFoods[0]}, ${favoriteFoods[1]}, and ${favoriteFoods[2]}!`);

console.log(100 == "100"); // true (loose comparison)

console.log(100 === "100"); // false (strict comparison)

let num = Number("42");

console.log(PI + num); // 45.14159