Variables and Data Types

Variables are containers for storing data values. In JavaScript, we have several ways to declare variables and multiple data types to work with.

Declaring Variables

JavaScript has three keywords for declaring variables:

let

Use let for variables that will change:

let score = 0;
score = 10; // ✓ Can be reassigned
score = 20; // ✓ Can be reassigned again

const

Use const for values that shouldn't change:

const PI = 3.14159;
PI = 3; // ✗ Error! Cannot reassign a const

var (Legacy)

var is the old way of declaring variables. Use let and const instead:

var oldWay = "Don't use this";

Data Types

JavaScript has several primitive data types:

Strings

Text values, enclosed in quotes:

const name = "Alice";
const greeting = 'Hello!';
const template = `Hello, ${name}!`; // Template literal

Numbers

Both integers and decimals:

const age = 25;
const price = 19.99;
const negative = -10;

Booleans

True or false values:

const isLoggedIn = true;
const hasAccess = false;

Undefined and Null

let notYetAssigned; // undefined
const empty = null; // intentionally empty

Type Checking

Use typeof to check a value's type:

console.log(typeof "hello"); // "string"
console.log(typeof 42);      // "number"
console.log(typeof true);    // "boolean"
console.log(typeof undefined); // "undefined"

String Operations

const firstName = "John";
const lastName = "Doe";

// Concatenation
const fullName = firstName + " " + lastName;

// Template literals (preferred)
const greeting = `Hello, ${firstName}!`;

// String methods
console.log(firstName.length);     // 4
console.log(firstName.toUpperCase()); // "JOHN"
console.log(fullName.includes("Doe")); // true

Number Operations

const a = 10;
const b = 3;

console.log(a + b);  // 13 (addition)
console.log(a - b);  // 7 (subtraction)
console.log(a * b);  // 30 (multiplication)
console.log(a / b);  // 3.333... (division)
console.log(a % b);  // 1 (remainder)
console.log(a ** b); // 1000 (exponentiation)

Practice Exercise

Try creating variables for your profile:

const myName = "Your Name";
const myAge = 25;
const isStudent = true;

console.log(`My name is ${myName}.`);
console.log(`I am ${myAge} years old.`);
console.log(`Am I a student? ${isStudent}`);

Summary

• Use const by default, let when you need to reassign
• JavaScript has strings, numbers, booleans, undefined, and null
• Template literals make string formatting easier
• Use typeof to check data types

In the next chapter, we'll learn about operators and how to manipulate data.