Variables & Types

Variables are how programs remember information. You store a value in a variable, give it a name, and refer to it later. In modern JavaScript, you'll mostly use let and const.

let and const

Use const when the value won't change, and let when it will:

const siteName = "Codecraft";   // can't be reassigned
let score = 0;                  // can be reassigned later

score = 10;                     // ✓ fine
siteName = "Other";             // ✗ TypeError

Data Types

JavaScript has several built-in data types:

// String — text, wrapped in quotes
const name = "Alice";
const greeting = 'Hello, world!';
const message = `Hello, ${name}!`;   // template literal

// Number — integers and decimals
const age = 25;
const price = 9.99;

// Boolean — true or false
const isLoggedIn = true;
const hasError = false;

// null — intentionally empty
const selectedItem = null;

// undefined — variable declared but not assigned
let currentUser;
console.log(currentUser);   // undefined

typeof

Use typeof to check what type a value is:

typeof "hello"      // "string"
typeof 42           // "number"
typeof true         // "boolean"
typeof undefined    // "undefined"
typeof null         // "object"  ← known quirk in JavaScript

String Operations

const first = "Jane";
const last = "Doe";

// Concatenation
const full = first + " " + last;   // "Jane Doe"

// Template literals (preferred)
const full2 = `${first} ${last}`;  // "Jane Doe"

// String length
console.log(full.length);          // 8

// Access a character
console.log(full[0]);              // "J"

// Methods
console.log(full.toUpperCase());   // "JANE DOE"
console.log(full.includes("Jane")); // true

Number Operations

const a = 10;
const b = 3;

console.log(a + b);   // 13
console.log(a - b);   // 7
console.log(a * b);   // 30
console.log(a / b);   // 3.333...
console.log(a % b);   // 1  (remainder)
console.log(a ** b);  // 1000 (exponent)

Arrays

Arrays store ordered lists of values:

const colors = ["red", "green", "blue"];

console.log(colors[0]);       // "red"
console.log(colors.length);   // 3

colors.push("yellow");        // add to end
colors.pop();                 // remove from end

Objects

Objects store key-value pairs:

const user = {
  name: "Alice",
  age: 25,
  isAdmin: false,
};

console.log(user.name);       // "Alice"
console.log(user["age"]);     // 25

user.age = 26;                // update a property