Variables and Data Types

Variables are the foundation of programming. Let's master them!

What is a Variable?

A variable is a "box" that stores a value.

javascript

let age = 25;//  ↑     ↑// name  value

let, const and var

let - Can be changed

javascript

let score = 0;score = 10;      // ✅ Worksscore = 100;     // ✅ Works

const - Cannot be changed

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const PI = 3.14159;PI = 3;          // ❌ Error!

var - Old way (avoid)

javascript

var name = "Old";  // Works, but prefer let

When to use what?

const by default (most cases)
let when the value will change
var never in new code

Data Types

String (text)

javascript

const name = "Maria";const phrase = 'Hello World';const template = `Hello, ${name}`; // Useful methodsname.length;           // 5name.toUpperCase();    // "MARIA"name.toLowerCase();    // "maria"name.includes("ri");   // true

Number

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const integer = 42;const decimal = 3.14;const negative = -10; // Operations5 + 3;    // 810 - 4;   // 63 * 4;    // 1210 / 2;   // 510 % 3;   // 1 (remainder) // Useful methodsMath.round(3.7);    // 4Math.floor(3.7);    // 3Math.ceil(3.2);     // 4Math.random();      // 0 to 1

Boolean (true/false)

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const active = true;const blocked = false; // Results of comparisons5 > 3;     // true10 < 5;    // false5 === 5;   // true5 !== 3;   // true

Undefined and Null

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let variable;           // undefined (declared but no value)const empty = null;     // null (intentionally empty)

Type Conversion

String to Number

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const text = "42";const number = Number(text);      // 42const number2 = parseInt("42");   // 42const number3 = parseFloat("3.14"); // 3.14

Number to String

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const number = 42;const text = String(number);      // "42"const text2 = number.toString();  // "42"

Checking Type

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typeof "Hello";  // "string"typeof 42;       // "number"typeof true;     // "boolean"typeof undefined; // "undefined"

Comparison Operators

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// Equality5 == "5";    // true (only value)5 === "5";   // false (value AND type) ← USE THIS! // Inequality5 != "5";    // false5 !== "5";   // true ← USE THIS! // Greater/Less5 > 3;       // true5 >= 5;      // true3 < 5;       // true3 <= 3;      // true

Important: Always use === and !==!

Logical Operators

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// AND (&&) - both truetrue && true;    // truetrue && false;   // false // OR (||) - at least one truetrue || false;   // truefalse || false;  // false // NOT (!) - inverts!true;           // false!false;          // true

Practical Exercise

javascript

// Interactive calculationconst name = prompt("What's your name?");const birthYear = Number(prompt("Year born?")); const age = 2026 - birthYear;const adultYear = birthYear + 18; console.log(`${name}, you are ${age} years old.`); if (age >= 18) {    console.log("You are an adult!");} else {    console.log(`You will be an adult in ${adultYear}.`);}

Summary

✅ const by default, let when it changes
✅ String, Number, Boolean, Undefined, Null
✅ Always === for comparison
✅ && (and), || (or), ! (not)

In the next lesson, we'll learn to make decisions with if/else! 🚀