JavaScript Function Parameters – Default, Rest, and Destructured

function introduce(firstName, lastName, age) {
  return `${firstName} ${lastName}, age ${age}`;
}

console.log(introduce("Alice", "Smith", 30)); // Alice Smith, age 30
console.log(introduce("Bob", "Jones", 25));   // Bob Jones, age 25

// Extra arguments are silently ignored
console.log(introduce("Alice", "Smith", 30, "extra")); // OK

// Missing arguments become undefined
console.log(introduce("Alice")); // Alice undefined, age undefined

function createUser(name, role = "viewer", isActive = true) {
  return { name, role, isActive };
}

console.log(createUser("Alice"));                    // {name:'Alice', role:'viewer', isActive:true}
console.log(createUser("Bob", "admin"));             // {name:'Bob', role:'admin', isActive:true}
console.log(createUser("Carol", "editor", false));   // {name:'Carol', role:'editor', isActive:false}

// Passing undefined explicitly triggers the default
console.log(createUser("Dave", undefined, false));   // {name:'Dave', role:'viewer', isActive:false}

// Passing null does NOT trigger the default
console.log(createUser("Eve", null, true));          // {name:'Eve', role:null, isActive:true}

// Variadic sum function
function sum(...numbers) {
  return numbers.reduce((acc, n) => acc + n, 0);
}

console.log(sum(1, 2, 3));          // 6
console.log(sum(1, 2, 3, 4, 5));    // 15
console.log(sum());                 // 0

// Mix: fixed params + rest
function logMessage(level, ...messages) {
  const formatted = messages.join(" ");
  console.log(`[${level.toUpperCase()}] ${formatted}`);
}

logMessage("info", "Server", "started", "on port 3000");
logMessage("error", "Connection", "refused");

// Legacy approach — avoid in modern code
function legacySum() {
  let total = 0;
  for (let i = 0; i < arguments.length; i++) {
    total += arguments[i];
  }
  return total;
}
console.log(legacySum(1, 2, 3)); // 6

// Modern rest parameter approach (preferred)
function modernSum(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}
console.log(modernSum(1, 2, 3)); // 6

// Object destructuring in parameters
function displayUser({ name, age, city = "Unknown" }) {
  console.log(`${name}, ${age} years old, from ${city}`);
}

displayUser({ name: "Alice", age: 30, city: "London" });
displayUser({ name: "Bob", age: 25 }); // city defaults to "Unknown"

// Array destructuring in parameters
function processCoords([x, y, z = 0]) {
  console.log(`x=${x}, y=${y}, z=${z}`);
}

processCoords([10, 20]);
processCoords([1, 2, 3]);

// Hard to read: what does each argument mean?
function createButton("Submit", true, "primary", "lg", null) { /* ... */ }

// Named parameters — self-documenting
function createButton({ text, disabled = false, variant = "default", size = "md", icon = null } = {}) {
  return { text, disabled, variant, size, icon };
}

const btn = createButton({ text: "Submit", variant: "primary", size: "lg" });
console.log(btn);

// Can call with no args because of = {} default
const defaultBtn = createButton();
console.log(defaultBtn);

// Primitive — pass by value (copy)
function double(n) {
  n = n * 2; // Modifies local copy only
  return n;
}
let x = 5;
double(x);
console.log(x); // Still 5 — original unchanged

// Object — pass by reference
function addScore(player) {
  player.score += 10; // Modifies original!
}
const p = { name: "Alice", score: 100 };
addScore(p);
console.log(p.score); // 110 — original changed!

// Avoid mutation: create and return new object
function addScoreImmutable(player) {
  return { ...player, score: player.score + 10 };
}
const p2 = { name: "Bob", score: 100 };
const p2Updated = addScoreImmutable(p2);
console.log(p2.score);        // 100 — original safe
console.log(p2Updated.score); // 110

// Higher-order function accepting a callback
function processItems(items, transform) {
  return items.map(transform);
}

const prices = [10, 20, 30];
const withTax = processItems(prices, price => price * 1.2);
console.log(withTax); // [12, 24, 36]

// Callback with error-first pattern (Node.js style)
function fetchData(url, onSuccess, onError) {
  if (!url) {
    onError(new Error("URL required"));
    return;
  }
  onSuccess({ data: "mock response from " + url });
}

fetchData("https://api.example.com",
  (res) => console.log("Got:", res.data),
  (err) => console.log("Error:", err.message)
);