JavaScript Basics for Beginners

Variables

JavaScript has three ways to declare variables:

const name = "Rust";   // can't be reassigned
let age = 5;           // can be reassigned
var old = "avoid";     // function-scoped, avoid using

Data Types

const str = "hello";        // string
const num = 42;             // number
const bool = true;          // boolean
const arr = [1, 2, 3];      // array
const obj = { key: "value" }; // object
const nothing = null;       // null
const undef = undefined;    // undefined

Functions

Function Declaration

function add(a, b) {
    return a + b;
}

Arrow Functions

const add = (a, b) => a + b;
const double = x => x * 2;

Control Flow

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

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

for (const item of items) {
    console.log(item);
}

DOM Manipulation

// Select elements
const heading = document.querySelector("h1");
const buttons = document.querySelectorAll("button");

// Modify content
heading.textContent = "New Title";
heading.style.color = "blue";

// Events
button.addEventListener("click", () => {
    alert("Clicked!");
});

Modern JavaScript Features

• Template literals: `Hello, ${name}!`
• Destructuring: const { name, age } = person;
• Spread operator: const copy = [...arr];
• Async/await: const data = await fetch(url);

JavaScript is everywhere — browser, server (Node.js), and even desktop apps.