Errors and Exceptions in TypeScript in TypeScript

spend(amount: number): void {
    if (amount > this.money) {
        throw new Error("not enough money");
    }

    this.money -= amount;
}

try {
    wallet.spend(80);
} catch (e) {
    console.log("error: " + (e as Error).message);
}

Full lesson:

Some requests can't be done. A wallet with 50 can't spend 80.

The method could return a special value, like -1. But the caller may forget to check it. Then the program goes on with a wrong number.

An exception is better. It is an error that the program can't miss.

Throwing an error

• throw new Error("not enough money") stops the method at once.
• The lines after it don't run. So the wallet keeps its money.
• Error is the built-in error type.
• The text in the parentheses is the error's message.

Catching it

• Put the call inside a try block.
• When the call throws, the rest of the try block is skipped.
• The catch (e) block runs instead. e is what was thrown.
• Any value can be thrown, so TypeScript doesn't know the type of e.
• e as Error tells it that e is an Error. Then .message is its message.
• After the catch block, the program goes on as usual.

An error that nobody catches stops the whole program. Node prints the error and the line where it happened.

Your task

A Wallet starts with 50. Its spend method takes money out and returns what is left.

• In spend, throw an Error when amount is more than this.money. Its message is "not enough money".
• In the loop, call spend inside a try block. Log what was spent and what is left.
• Catch the error. Log error: and its message.

The loop tries to spend 20, then 40, then 30.

Expected output:

spent 20, 30 left
error: not enough money
spent 30, 0 left