Making Decisions with Conditionals

Programs need to make decisions. Let's learn how!

if - Simple Decision

javascript

const age = 18; if (age >= 18) {    console.log("You are an adult!");}

Structure:

javascript

if (condition) {    // code executes if condition is TRUE}

if/else - Two Options

javascript

const age = 15; if (age >= 18) {    console.log("You are an adult!");} else {    console.log("You are a minor.");}

else if - Multiple Options

javascript

const grade = 75; if (grade >= 90) {    console.log("A");} else if (grade >= 80) {    console.log("B");} else if (grade >= 70) {    console.log("C");} else if (grade >= 60) {    console.log("D");} else {    console.log("F");}

Compound Conditions

AND (&&) - Both true

javascript

const age = 25;const hasLicense = true; if (age >= 18 && hasLicense) {    console.log("You can drive!");}

OR (||) - At least one true

javascript

const day = "Saturday"; if (day === "Saturday" || day === "Sunday") {    console.log("It's the weekend!");}

NOT (!) - Inverts

javascript

const loggedIn = false; if (!loggedIn) {    console.log("Please log in.");}

Ternary Operator

Shortcut for simple if/else:

javascript

const age = 20; // With if/elselet status;if (age >= 18) {    status = "Adult";} else {    status = "Minor";} // With ternaryconst status = age >= 18 ? "Adult" : "Minor";

Structure: condition ? ifTrue : ifFalse

switch - Multiple Exact Values

javascript

const day = 3; switch (day) {    case 1:        console.log("Monday");        break;    case 2:        console.log("Tuesday");        break;    case 3:        console.log("Wednesday");        break;    case 4:        console.log("Thursday");        break;    case 5:        console.log("Friday");        break;    default:        console.log("Weekend");}

Important: Don't forget the break!

Truthy and Falsy

In JavaScript, some values are considered "false":

javascript

// Falsy values (become false)false0"" (empty string)nullundefinedNaN // Everything else is truthy"text"    // true42        // true[]        // true (empty array!){}        // true (empty object!)

Useful example:

javascript

const name = ""; if (name) {    console.log(`Hello, ${name}!`);} else {    console.log("Hello, visitor!");}

Practical Exercise: Calculator

javascript

const num1 = Number(prompt("First number:"));const operator = prompt("Operator (+, -, *, /):");const num2 = Number(prompt("Second number:")); let result; if (operator === "+") {    result = num1 + num2;} else if (operator === "-") {    result = num1 - num2;} else if (operator === "*") {    result = num1 * num2;} else if (operator === "/") {    if (num2 === 0) {        result = "Error: cannot divide by zero!";    } else {        result = num1 / num2;    }} else {    result = "Invalid operator!";} alert(`Result: ${result}`);

Summary

✅ if for simple decisions
✅ if/else for two options
✅ else if for multiple options
✅ && (and), || (or), ! (not) for compound conditions
✅ Ternary for simple cases
✅ switch for many exact values

In the next lesson, we'll learn loops to repeat code! 🚀