What is JavaScript? Notes by home academy

 

What is JavaScript?

• JavaScript is a lightweight, interpreted scripting language used to make webpages interactive.

• Created by Brendan Eich in 1995.

• It runs in the browser (client-side) but can also run on servers (Node.js).

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📌 Key Features of JavaScript

Feature	Description
Client-Side	Runs in the user's browser
Interpreted	No need to compile
Dynamic Typing	No need to declare variable type
Object-Based	Uses objects for structure
Event-Driven	Responds to user actions (click, type)
Cross-Platform	Works on all browsers

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🧱 Basic Syntax

javascript
<script>
  alert("Hello, JavaScript!");
</script>

JavaScript is embedded within an HTML file using the <script> tag.

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📚 Basic Concepts with Examples

1. Variables

javascript
var x = 10;       // old way
let y = 20;       // block scoped
const z = 30;     // constant

2. Data Types

javascript
let name = "Ali";       // String
let age = 25;           // Number
let isAdult = true;     // Boolean
let user = null;        // Null
let temp;               // Undefined

3. Operators

javascript
+  -  *  /  %       // Arithmetic
== === != !==       // Comparison
&& || !             // Logical

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🔁 Control Structures

If-else

javascript
if (age > 18) {
  console.log("Adult");
} else {
  console.log("Minor");
}

Switch

javascript
switch(day) {
  case 1: alert("Monday"); break;
  default: alert("Other");
}

Loops

javascript
for (let i = 0; i < 5; i++) {
  console.log(i);
}

let i = 0;
while (i < 5) {
  console.log(i);
  i++;
}

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🔤 Functions in JavaScript

javascript
function greet(name) {
  return "Hello " + name;
}

console.log(greet("Afzal"));

Arrow Function:

javascript
const greet = (name) => "Hi " + name;

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🎯 Events in JavaScript

Event	Description
onclick	When element is clicked
onchange	Value changed
onload	When page loads
onmouseover	Mouse over element

Example:

html
<button onclick="alert('Clicked!')">Click Me</button>

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📦 JavaScript and DOM (Document Object Model)

DOM allows JavaScript to interact with HTML elements.

html
<p id="demo">Hello</p>
<script>
  document.getElementById("demo").innerHTML = "Changed Text";
</script>

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⚙️ Arrays and Objects

Arrays

javascript
let fruits = ["Apple", "Banana", "Mango"];
console.log(fruits[1]);  // Banana

Objects

javascript
let person = {
  name: "Ali",
  age: 25,
  city: "Srinagar"
};
console.log(person.name);  // Ali

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📐 Useful JavaScript Methods

Method	Use
alert()	Show popup message
console.log()	Print to browser console
prompt()	Input box popup
parseInt()	Convert to integer
toString()	Convert to string

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🌍 Practical Applications of JavaScript

Area	Examples
Web Interactivity	Click, hover, drag/drop
Form Validation	Check required fields
Animations	Interactive sliders, menus
Web APIs	Google Maps, YouTube APIs
Server-side (Node.js)	Real-time apps, chatbots

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❓ Frequently Asked MCQs

1. Who developed JavaScript?
   a) Dennis Ritchie
   b) Bjarne Stroustrup
   c) Brendan Eich ✅
   d) Tim Berners-Lee

2. Which tag is used to write JavaScript in HTML?
   a) <js>
   b) <javascript>
   c) <script> ✅
   d) <code>

3. Which is NOT a JavaScript data type?
   a) String
   b) Number
   c) Float ✅ (JS has only Number type)
   d) Boolean

4. Which method displays a message in a popup box?
   a) print()
   b) popup()
   c) alert() ✅
   d) show()

5. What is the result of 2 + "2" in JavaScript?
   a) 4
   b) 22 ✅ (String concatenation)
   c) NaN
   d) Error

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🧠 Important Points for Exams

• JavaScript is case-sensitive

• == checks value only, === checks value + type

• Can be used for both client-side and server-side

• Commonly used with HTML + CSS

• JavaScript files use .js extension

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📝 Conclusion

JavaScript makes websites dynamic and interactive. It's essential for frontend development, and with Node.js, for backend as well.