Challenge 2 — Solution

Task: Define a custom error class InsufficientFundsError extending
Error, with a constructor taking (message, shortfall) and storing
shortfall. Write a function withdraw(balance, amount) that throws
it if amount > balance, including the shortfall (amount - balance).
Catch it and log a message using error.shortfall.

class InsufficientFundsError extends Error {
    constructor(message, shortfall) {
        super(message);
        this.name = "InsufficientFundsError";
        this.shortfall = shortfall;
    }
}

function withdraw(balance, amount) {
    if (amount > balance) {
        throw new InsufficientFundsError(
            "Not enough funds to complete withdrawal",
            amount - balance
        );
    }
    return balance - amount;
}

try {
    withdraw(100, 150);
} catch (error) {
    console.log(`${error.message}. You are short by ${error.shortfall}.`);
}


Expected output:

Not enough funds to complete withdrawal. You are short by 50.


Notes:
- super(message) is required before this.name/this.shortfall can be
  set — it's what makes InsufficientFundsError behave like a real
  Error (with a proper message and stack trace) rather than just an
  ordinary object.
- shortfall is calculated once, at the point the error is thrown
  (amount - balance), and travels along with the error itself —
  no separate calculation is needed inside the catch block.
- error instanceof InsufficientFundsError (not used here, but worth
  knowing) would let a catch block branch differently for this
  specific error type versus any other.