meet it

Closures let you hide data behind a few safe functions.

Any line of code can change a plain object's properties, including a buggy one. A closure can keep a value instead: only the functions made inside the factory can reach it.

Here the factory returns an object holding two functions. They share one hidden balance. The rest of the program must go through them, and deposit can refuse bad amounts. This idea, data plus its guards and nothing else on show, is .

function makeWallet() {
  let balance = 0;
  return {
    deposit(amount) {
      if (amount <= 0) { return "Refused"; }
      balance = balance + amount;
      return "OK";
    },
    getBalance() { return balance; }
  };
}
const wallet = makeWallet();
console.log(wallet.deposit(20), wallet.deposit(-5), wallet.getBalance());
console.log(wallet.balance);

Inside an object, deposit(amount) { } is short for deposit: function (amount) { }.

guess first

Someone tries to cheat. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

function makeScore() {
  let points = 0;
  return {
    add() { points = points + 10; },
    show() { return points; }
  };
}
const score = makeScore();
score.add();
score.points = 500;
console.log(score.show());

your turn

Your turn. Guard the muffin stock.

In the first blank, compare against the private stock count so sell refuses big orders. In the second, choose a starting stock of at least 4.

Your code

press run to see what happens

fix it

The coin jar never fills up. Can you fix it?

Broken code to fix

press run to see what happens

coins, copies the number into the object once, when the jar is made. Does that copy ever update?
The object needs a function that reads the private coins every time it is asked.
Replace coins, with count() { return coins; }, and use jar.count() on the last line.

nailed it

your data has a lock on it now. :D

Variables inside a factory are private to its functions.
Return only the functions the rest of the program needs.
Guards inside those functions can refuse bad changes.

new words in your collection

private
encapsulation

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