Lesson 18 — Module pattern

Public API, private data

The module pattern uses a function that runs immediately (an IIFE — immediately invoked function expression, written (() => { … })()). Variables inside it are private; the object it returns is the public API — the only way to use them. Closures make the returned functions keep access to the private variables.

const bank = (() => {
  let balance = 0;              // private
  return { deposit: (n) => (balance += n) };
})();
console.log(bank.balance);      // undefined — hidden

Why it matters: hiding internals means nobody can break your data by accident — the same idea behind real modules and classes.