LESSON 406 OF 410 · PROJECTS: OBJECT-ORIENTED

Bank Account - Protect the Balance in JavaScript

Practice

Build a BankAccount class that keeps its balance private and lets other code change it only through methods that check the rules. This is encapsulation: the object protects its own data.

Private balance. Declare the balance as a truly private field with the # prefix, #balance. Code outside the class cannot read or write it, and inside you reach it as this.#balance.

What the class must have (JavaScript):

constructor(initialBalance): creates an account that starts with the given balance.
deposit(amount): adds amount to the balance. If amount is zero or negative, do nothing (no error, the balance stays the same).
withdraw(amount): if amount is zero or negative, or larger than the balance, change nothing and return false. Otherwise subtract it from the balance and return true. Withdrawing exactly the whole balance is allowed (the balance becomes 0).
getBalance(): returns the current balance. It is the only way for outside code to read it.

The driver. The driver at the end of the starter code is already written and uses your class. It runs these checks:

let account = new BankAccount(100);
account.deposit(50);
account.withdraw(30);
account.withdraw(200);
console.log("Balance: " + account.getBalance());
account.deposit(-20);
account.deposit(0);
account.withdraw(-5);
console.log("Balance: " + account.getBalance());
if (account.withdraw(120)) {
    console.log("Withdrew all");
}
console.log("Balance: " + account.getBalance());
if (!account.withdraw(1)) {
    console.log("Rejected");
}

Walk through it: start with 100, deposit 50 (150), withdraw 30 (120), withdraw 200 is refused (still 120). Then deposit -20, deposit 0 and withdraw -5 are all ignored (still 120). Withdrawing 120 succeeds and empties the account, so the balance is 0, and withdrawing 1 more is refused.

Expected output:

Balance: 120
Balance: 120
Withdrew all
Balance: 0
Rejected

Both deposit and withdraw start with a guard that rejects amount <= 0. Write the guard first, then the change to the balance.

Declare #balance; in the class body, then in the constructor write this.#balance = initialBalance;.

if (amount <= 0 || amount > this.#balance) return false;, then this.#balance -= amount; and return true;.

class BankAccount {
    #balance;

    constructor(initialBalance) {
        this.#balance = initialBalance;
    }

    deposit(amount) {
        if (amount <= 0) return;
        this.#balance += amount;
    }

    withdraw(amount) {
        if (amount <= 0 || amount > this.#balance) return false;
        this.#balance -= amount;
        return true;
    }

    getBalance() {
        return this.#balance;
    }
}

let account = new BankAccount(100);
account.deposit(50);
account.withdraw(30);
account.withdraw(200);
console.log("Balance: " + account.getBalance());
account.deposit(-20);
account.deposit(0);
account.withdraw(-5);
console.log("Balance: " + account.getBalance());
if (account.withdraw(120)) {
    console.log("Withdrew all");
}
console.log("Balance: " + account.getBalance());
if (!account.withdraw(1)) {
    console.log("Rejected");
}

STEP 1 OF 13

Guided

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Bank Account - Protect the Balance in JavaScript

Bank Account in JavaScript