JavaScript Basics

1. Variables

Variables store data values. Use let, const, or var to declare them.

let name = "John"; // Mutable variable
const age = 30; // Immutable variable
var city = "New York"; // Old-style variable

2. Data Types

JavaScript has the following data types:

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

let str = "Hello"; // string
let num = 42; // number
let isHappy = true; // boolean
let nothing = null; // null
let notDefined; // undefined
let obj = { key: "value" }; // object
let arr = [1, 2, 3]; // array

3. Operators

• Arithmetic: +, -, *, /, %
• Comparison: ==, ===, !=, !==, >, <, >=, <=
• Logical: &&, ||, !

console.log(10 + 5); // 15
console.log(10 === "10"); // false (strict equality)

4. Control Structures

• If-Else

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

• Loops

for (let i = 0; i < 5; i++) {
  console.log(i); // 0, 1, 2, 3, 4
}

let i = 0;
while (i < 3) {
  console.log(i); // 0, 1, 2
  i++;
}

5. Functions

function greet(name) {
  return `Hello, ${name}!`;
}
console.log(greet("Alice")); // "Hello, Alice!"

6. Events

Interact with the DOM using events.

document.querySelector("button").addEventListener("click", function () {
  alert("Button clicked!");
});

7. ES6+ Features

• Arrow Functions

const add = (a, b) => a + b;
console.log(add(3, 5)); // 8

• Template Literals

const message = `Age: ${age}`;
console.log(message); // "Age: 30"

• Destructuring

const user = { name: "John", age: 30 };
const { name, age } = user;
console.log(name); // "John"

• Modules

// Export
export const greet = () => "Hello";
// Import
import { greet } from "./file.js";

8. Objects and Arrays

• Objects

const person = { firstName: "John", lastName: "Doe" };
console.log(person.firstName); // "John"

• Arrays

const colors = ["red", "green", "blue"];
colors.push("yellow");
console.log(colors); // ["red", "green", "blue", "yellow"]

9. Promises and Async/Await

Handle asynchronous operations.

const fetchData = async () => {
  try {
    const response = await fetch("https://jsonplaceholder.typicode.com/users");
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error(error);
  }
};

10. Classes

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    console.log(`${this.name} makes a sound.`);
  }
}
const dog = new Animal("Dog");
dog.speak(); // "Dog makes a sound."