What are the Module Pattern and the Revealing Module Pattern?

Pre-ES-modules patterns for creating encapsulated, private state in JavaScript.

// Module Pattern
const counter = (function() {
  let _count = 0; // private — inaccessible from outside

  return {
    increment() { _count++; },
    decrement() { _count--; },
    getCount()  { return _count; }
  };
})();

counter.increment();
counter.getCount(); // 1
counter._count;     // undefined — truly private ✓

// Revealing Module Pattern
// Define everything privately, then reveal selectively
const bankAccount = (function() {
  let _balance = 1000;
  let _transactions = [];

  function _log(type, amount) {
    _transactions.push({ type, amount, date: Date.now() });
  }

  function deposit(amount) {
    if (amount > 0) { _balance += amount; _log('deposit', amount); }
  }

  function withdraw(amount) {
    if (amount > 0 && amount <= _balance) {
      _balance -= amount; _log('withdrawal', amount);
    }
  }

  function getBalance() { return _balance; }
  function getHistory() { return [..._transactions]; }

  // Explicitly reveal the public interface
  return { deposit, withdraw, getBalance, getHistory };
})();