JS Operators: Arithmetic, Logical & Comparison

const a = 15;
const b = 4;
 
console.log(a + b);  // 19  (addition)
console.log(a - b);  // 11  (subtraction)
console.log(a * b);  // 60  (multiplication)
console.log(a / b);  // 3.75 (division, always returns float)
console.log(a % b);  // 3   (modulo/remainder)
console.log(a ** b); // 50625 (exponentiation, 15^4)

// Check if a number is even or odd
const num = 7;
const isEven = num % 2 === 0; // false (7 % 2 === 1)
 
// Calculate percentage
const total = 250;
const part = 75;
const percentage = (part / total) * 100; // 30
 
// Round to 2 decimal places
const price = 19.99 * 1.08; // 21.5892
const rounded = Math.round(price * 100) / 100; // 21.59
 
// Clamp a value between min and max
const value = 150;
const clamped = Math.min(Math.max(value, 0), 100); // 100

let count = 5;
 
// Postfix: returns current value, THEN increments
console.log(count++); // 5 (returns 5, count becomes 6)
console.log(count);   // 6
 
// Prefix: increments FIRST, then returns new value
console.log(++count); // 7 (count becomes 7, returns 7)
console.log(count);   // 7

console.log(5 > 3);   // true
console.log(5 < 3);   // false
console.log(5 >= 5);  // true
console.log(5 <= 4);  // false
console.log(5 === 5); // true (strict equality)
console.log(5 !== 3); // true (strict inequality)

// Strict equality (===): no type coercion
console.log(5 === 5);     // true
console.log(5 === "5");   // false (different types)
console.log(0 === false); // false (different types)
console.log(null === undefined); // false
 
// Loose equality (==): coerces types before comparing
console.log(5 == "5");    // true (string coerced to number)
console.log(0 == false);  // true (false coerced to 0)
console.log("" == false); // true (both coerce to 0)
console.log(null == undefined); // true (special rule)

console.log("apple" < "banana");  // true ("a" < "b" in Unicode)
console.log("abc" < "abd");      // true ("c" < "d")
console.log("10" < "9");         // true ("1" < "9" in Unicode, string comparison!)
console.log(10 < 9);             // false (numeric comparison)
 
// Case-insensitive comparison
const a = "Hello";
const b = "hello";
console.log(a.toLowerCase() === b.toLowerCase()); // true

console.log(true && true);     // true
console.log(true && false);    // false
console.log(false && true);    // false (short-circuits, doesn't evaluate right side)
 
// Short-circuit evaluation for conditional execution
const user = { name: "Ada", isAdmin: true };
user.isAdmin && console.log("Admin panel loaded"); // Logs the message
 
// Returns the actual value, not just true/false
console.log("hello" && 42);     // 42 (both truthy, returns last)
console.log("" && 42);          // "" (first is falsy, returns it)
console.log(null && "hello");   // null (first is falsy, returns it)

console.log(true || false);    // true
console.log(false || true);    // true
console.log(false || false);   // false
 
// Default values pattern
const username = "" || "Anonymous";  // "Anonymous" ("" is falsy)
const count = 0 || 10;              // 10 (0 is falsy, PROBLEM if 0 is valid!)
 
// Returns the actual value
console.log("hello" || "world"); // "hello" (first is truthy)
console.log("" || "fallback");   // "fallback"
console.log(null || undefined || "last"); // "last"

console.log(0 ?? 10);          // 0 (0 is NOT null/undefined)
console.log("" ?? "fallback"); // "" (empty string is NOT null/undefined)
console.log(null ?? "default"); // "default"
console.log(undefined ?? 42);  // 42
 
// Compare with ||
const score1 = 0 || 100;   // 100 (wrong! 0 was a valid score)
const score2 = 0 ?? 100;   // 0 (correct! ?? only catches null/undefined)
 
const theme1 = "" || "dark";  // "dark" (wrong if "" means "no theme selected")
const theme2 = "" ?? "dark";  // "" (correct! ?? preserves empty string)

console.log(!true);      // false
console.log(!false);     // true
console.log(!0);         // true (0 is falsy)
console.log(!"hello");   // false ("hello" is truthy)
console.log(!null);      // true
 
// Double-NOT for explicit boolean conversion
console.log(!!0);        // false
console.log(!!"hello");  // true
console.log(!!null);     // false
console.log(!!{});       // true

let x = 10;
 
x += 5;   // x = x + 5  → 15
x -= 3;   // x = x - 3  → 12
x *= 2;   // x = x * 2  → 24
x /= 4;   // x = x / 4  → 6
x %= 4;   // x = x % 4  → 2
x **= 3;  // x = x ** 3 → 8
 
// Logical assignment operators (ES2021)
let a = null;
a ??= "default"; // a = a ?? "default" → "default"
 
let b = "";
b ||= "fallback"; // b = b || "fallback" → "fallback"
 
let c = "existing";
c &&= "updated";  // c = c && "updated" → "updated"

// Multiplication before addition (standard math order)
console.log(2 + 3 * 4);     // 14, not 20
console.log((2 + 3) * 4);   // 20 (parentheses override precedence)
 
// Comparison before logical AND
console.log(5 > 3 && 2 < 4); // true (comparisons run first, then &&)
 
// Logical AND before logical OR
console.log(true || false && false); // true (&& runs first: false && false = false, then true || false = true)

function validateRegistration(email, password, age) {
  const isEmailValid = email.includes("@") && email.includes(".");
  const isPasswordStrong = password.length >= 8 && /[A-Z]/.test(password) && /\d/.test(password);
  const isAgeValid = typeof age === "number" && age >= 13 && age <= 120;
 
  if (!isEmailValid) return { valid: false, error: "Invalid email format" };
  if (!isPasswordStrong) return { valid: false, error: "Password needs 8+ chars, uppercase, and digit" };
  if (!isAgeValid) return { valid: false, error: "Age must be 13-120" };
 
  return { valid: true, error: null };
}
 
const result = validateRegistration("ada@runehub.dev", "Str0ngPass", 25);
console.log(result); // { valid: true, error: null }

const config = {
  database: {
    host: "localhost",
    port: 5432,
  },
};
 
// Optional chaining (?.) with nullish coalescing (??)
const dbHost = config?.database?.host ?? "127.0.0.1";
const dbPort = config?.database?.port ?? 3306;
const cacheHost = config?.cache?.host ?? "localhost"; // "localhost" (cache doesn't exist)
 
console.log(dbHost);   // "localhost"
console.log(dbPort);   // 5432
console.log(cacheHost); // "localhost"

function getStatusBadge(user) {
  const isActive = user.lastLogin && Date.now() - new Date(user.lastLogin) < 86400000;
  const isPremium = user.subscription === "premium" || user.subscription === "enterprise";
  const isVerified = !!user.emailVerifiedAt;
 
  return {
    text: isActive ? "Online" : "Offline",
    color: isPremium ? "gold" : "gray",
    icon: isVerified ? "check" : "alert",
  };
}