Your port of parseAmount() now refuses Joan’s refund on line 6 with throw new Error(). The refund is only one bad row, though. Number("n/a") and Number("1,250.00") don’t throw an error the way float() does, so those rows slip into the total, and the trustees’ report reads £NaN.
In this lesson, you’ll pick the error type that fits a bad amount and give it a message that says what arrived, so Joan finds out about rows like those.
Look Inside a Thrown Error
In Python, an exception’s class says what kind of problem it is. Joan’s parse_amount() raises a ValueError for the refund, and a handler can read the kind and the message:
def parse_amount(text):
amount = float(text)
if amount <= 0:
raise ValueError(f"amount must be positive, got {amount}")
return amount
try:
parse_amount("-40.00")
except ValueError as err:
print(type(err).__name__)
print(err)
Python prints ValueError, then amount must be positive, got -40.0. A JavaScript error is an instance of a class too. Here, it’s RangeError, the built-in type the next section picks for a bad amount. Like every built-in error, it carries its class’s name as a string in .name:
function parseAmount(text) {
const amount = Number(text);
if (amount <= 0) {
throw new RangeError(`amount must be positive, got ${amount}`);
}
return amount;
}
try {
parseAmount("-40.00");
} catch (err) {
console.log(err.name);
console.log(err.message);
}
parseAmount("-40.00");
The last call has no catch, so it also shows how an uncaught error prints:
