Use getters and setters

Bad:

function makeBankAccount() {
  // ...

  return {
    balance: 0
    // ...
  };
}

const account = makeBankAccount();
account.balance = 100;

Good:

function makeBankAccount() {
  // this one is private
  let balance = 0;

  // a "getter", made public via the returned object below
  function getBalance() {
    return balance;
  }

  // a "setter", made public via the returned object below
  function setBalance(amount) {
    // ... validate before updating the balance
    balance = amount;
  }

  return {
    // ...
    getBalance,
    setBalance
  };
}

const account = makeBankAccount();
account.setBalance(100);

Using getters and setters to access data on objects could be better than simply looking for a property on an object. “Why?” you might ask. Well, here’s an unorganized list of reasons why:

When you want to do more beyond getting an object property, you don’t have to look up and change every accessor in your codebase.
Makes adding validation simple when doing a set.
Encapsulates the internal representation.
Easy to add logging and error handling when getting and setting.
You can lazy load your object’s properties, let’s say getting it from a server.