JavaScript Lesson 5: Conditions

Conditions in JavaScript work similarly to Python but use curly braces instead of indentation. The logic is the same.

if / else if / else

const hour = 14; // 2 PM

if (hour < 12) {
  console.log("Good morning!");
} else if (hour < 18) {
  console.log("Good afternoon!");
} else {
  console.log("Good evening!");
}

Switch Statement

const day = "Monday";

switch (day) {
  case "Monday":
  case "Tuesday":
  case "Wednesday":
  case "Thursday":
  case "Friday":
    console.log("Weekday - time to work!");
    break;
  case "Saturday":
  case "Sunday":
    console.log("Weekend - time to relax!");
    break;
  default:
    console.log("Invalid day");
}

Truthy and Falsy

// Falsy values (behave like false in conditions):
// false, 0, -0, "", null, undefined, NaN

// Truthy: everything else

const userName = "";

if (userName) {
  console.log("Welcome,", userName);
} else {
  console.log("Please enter your name");
}

// Useful patterns:
const items = [];
if (items.length) {
  console.log("Has items");
} else {
  console.log("Empty!");
}

Nullish Coalescing (??)

// ?? returns right side only if left is null or undefined
const user = null;
const name = user ?? "Guest";     // "Guest"

const score = 0;
const points = score ?? 100;       // 0 (not 100!)
// Because ?? only triggers for null/undefined, not 0

// Contrast with ||:
const pts2 = score || 100;         // 100 (0 is falsy!)