Lecture 5 – Introduction to JavaScript

Coding with JavaScript. Learn variables, data types, operators, functions, conditionals, and DOM manipulation with practical examples and mini labs.

Introduction

JavaScript (JS) makes web pages interactive. With JS you can read user input, react to clicks, change the page without reloading, and talk to servers. In this lecture you’ll master variables, data types, operators, functions, conditionals, and DOM manipulation.

Variables, Data Types, Operators

Variables

Use let and const (modern, block-scoped). Avoid var for now.

js
const siteName = "ElecturesAI"; // constant
let visitors = 0;               // can change later
visitors += 1;

When to use which?

• const by default;
• let when you must reassign.

Data Types

• Primitive: string, number, boolean, null, undefined, bigint, symbol
• Reference: object, array, function, Date, etc.

js
const title = "JS Intro";        // string
const price = 499.99;            // number
const isPublished = true;        // boolean
let notSet;                      // undefined
const tags = ["js","basics"];    // array (object)
const author = {name:"NexAI"};   // object

Operators (quick hits)

• Arithmetic: + - * / % **
• Assignment: = += -=
• Comparison: === !== < > <= >=
• Logical: && || !
• String concat: + or template literal `Hi ${name}`

js
const a = 7, b = 3;
const sum = a + b;          // 10
const both = a > 5 && b < 5 // true
const msg = `Sum is ${sum}` // "Sum is 10"

Functions and Conditionals

Functions

Two common styles:

// Function declaration
function greet(name) {
  return `Hello, ${name}`;
}

// Arrow function
const square = n => n * n;

console.log(greet("Student"));
console.log(square(5)); // 25

Parameters & default values

function priceAfterDiscount(price, off = 0.1) {
  return price * (1 - off);
}

Conditionals

const score = 78;

if (score >= 90) {
  console.log("A");
} else if (score >= 80) {
  console.log("B");
} else if (score >= 70) {
  console.log("C");
} else {
  console.log("Keep going!");
}

// Ternary (short form)
const statusText = score >= 50 ? "Pass" : "Fail";

Common pitfalls

• Use === (strict equality) instead of == to avoid type coercion surprises.
• Wrap boolean logic for readability, e.g., (isLoggedIn && isAdmin).

DOM Manipulation (Making the Page React)

The DOM (Document Object Model) lets JS read/change HTML & CSS at runtime.

Selecting Elements

<h1 id="title">Hello</h1>
<button class="btn">Change Text</button>

const title = document.getElementById("title");
const button = document.querySelector(".btn");

Changing Content & Styles

title.textContent = "Welcome to JavaScript";
title.style.color = "#2f855a";

Responding to Events

button.addEventListener("click", () => {
  title.textContent = "You clicked the button!";
});

Creating & Appending Elements

const list = document.createElement("ul");
["HTML","CSS","JS"].forEach(item => {
  const li = document.createElement("li");
  li.textContent = item;
  list.appendChild(li);
});
document.body.appendChild(list);

Mini Lab Interactive Counter

HTML

<h2 id="count">0</h2>
<button id="inc">+</button>
<button id="dec">-</button>
<button id="reset">Reset</button>

CSS

h2 { font: 700 2rem/1.1 system-ui; color:#2f855a; }
button { margin:.25rem; padding:.5rem 1rem; }

JavaScript

const countEl = document.getElementById("count");
let count = 0;

const render = () => (countEl.textContent = count);

document.getElementById("inc").addEventListener("click", () => { count++; render(); });
document.getElementById("dec").addEventListener("click", () => { count--; render(); });
document.getElementById("reset").addEventListener("click", () => { count = 0; render(); });

render();

The approach followed at E Lectures reflects both academic depth and easy-to-understand explanations.

Summary

• Use const/let, know your data types and operators.
• Write functions to reuse logic; control flow with if/else & ternary.
• Read and update the DOM; handle events to react to user actions.
  These skills power almost every interactive feature you’ll build next.

People also ask:

0 == "0"   // true
0 === "0"  // false

typeof 42;           // "number"
Array.isArray([]);   // true

const add = (a,b)=>a+b;  // arrow
function add2(a,b){return a+b;} // declaration