Practice Questions — Closures, Scope, and Hoisting

function makeCounter(start) {
  let count = start || 0;
  return {
    increment: function() { return ++count; },
    decrement: function() { return --count; },
    getCount: function() { return count; },
    reset: function() { count = start || 0; return count; }
  };
}

const c = makeCounter(0);
console.log(c.increment()); // 1
console.log(c.increment()); // 2
console.log(c.increment()); // 3
console.log(c.decrement()); // 2
console.log(c.getCount());  // 2
console.log(c.reset());     // 0
console.log(c.count);       // undefined (private!)

function once(fn) {
  let called = false;
  let result;
  return function() {
    if (!called) {
      result = fn.apply(this, arguments);
      called = true;
    }
    return result;
  };
}

const addOnce = once(function(a, b) {
  console.log("Computing...");
  return a + b;
});
console.log(addOnce(2, 3)); // Computing... 5
console.log(addOnce(5, 5)); // 5 (no Computing...)
console.log(addOnce(10, 10)); // 5 (still cached)

function createRateLimiter(fn, limit) {
  let lastCalled = 0;
  return function() {
    const now = Date.now();
    if (now - lastCalled >= limit) {
      lastCalled = now;
      return fn.apply(this, arguments);
    }
    console.log("Rate limited, try again later");
    return undefined;
  };
}

const limitedLog = createRateLimiter(function(msg) {
  console.log("Logged: " + msg);
}, 1000);

limitedLog("hello");  // Logged: hello
limitedLog("world");  // Rate limited
// After 1 second:
// limitedLog("again"); // Logged: again

function createCollection() {
  const items = [];
  return {
    add: function(item) {
      if (!items.includes(item)) {
        items.push(item);
        return true;
      }
      return false;
    },
    remove: function(item) {
      const idx = items.indexOf(item);
      if (idx !== -1) {
        items.splice(idx, 1);
        return true;
      }
      return false;
    },
    has: function(item) { return items.includes(item); },
    getAll: function() { return [...items]; },
    size: function() { return items.length; }
  };
}

const col = createCollection();
console.log(col.add("Aarav"));   // true
console.log(col.add("Priya"));   // true
console.log(col.add("Aarav"));   // false (duplicate)
console.log(col.has("Aarav"));   // true
console.log(col.remove("Aarav")); // true
console.log(col.size());          // 1

// BUGGY CODE:
// for (var i = 0; i < 5; i++) {
//   buttons[i].addEventListener('click', function() { console.log(i); });
// }

// FIX 1: Use let
for (let i = 0; i < 5; i++) {
  buttons[i].addEventListener('click', function() { console.log(i); });
}

// FIX 2: IIFE
for (var i = 0; i < 5; i++) {
  (function(j) {
    buttons[j].addEventListener('click', function() { console.log(j); });
  })(i);
}

// FIX 3: Closure function
function createHandler(index) {
  return function() { console.log(index); };
}
for (var i = 0; i < 5; i++) {
  buttons[i].addEventListener('click', createHandler(i));
}

function pipe() {
  const fns = Array.from(arguments);
  return function(input) {
    return fns.reduce(function(result, fn) {
      return fn(result);
    }, input);
  };
}

function double(x) { return x * 2; }
function addOne(x) { return x + 1; }
function square(x) { return x * x; }

const transform = pipe(double, addOne, square);
console.log(transform(3)); // 49 (3*2=6, +1=7, 7^2=49)
console.log(transform(5)); // 121 (5*2=10, +1=11, 11^2=121)

const simple = pipe(double, double);
console.log(simple(4)); // 16 (4*2=8, 8*2=16)