Checkpoint: Patterns for Real Codebases

Five questions on this section. Answer all of them, then see your score and why each answer is right or wrong.

What is wrong with this code?

TypeScript

const id = JSON.parse(body).orderId as OrderId;
cancelOrder(id);

Nothing; the brand is checked when cancelOrder runs.
JSON.parse always returns a string, so orderId is a string.
The cast brands a value nobody validated; parse it with the smart constructor, such as toOrderId(…), instead.
Branded types cannot be used with values that came from JSON.

A caller of charge(): Result<Receipt, PaymentError> writes if (result.error) showError(result.error). What happens?

It does not compile, because error only exists on the failure member; check result.ok first.
It compiles and works, because error is undefined on success.
It compiles, but the condition is always false.
It throws at runtime when the result is a success.

Cartwheel’s server renders the initial cart into the page as JSON. Bad data there means a broken deploy, not something a customer can fix. How should the client handle it?

A type predicate in an if, falling back to an empty cart when it fails.
const cart = JSON.parse(text) as Cart.
A parser returning a Result, with a retry when it fails.
An assertion function called once at startup that throws a clear error, which your error reporting picks up.

What is the problem with this emitter method?

TypeScript

on(event: keyof Events, handler: (payload: Events[keyof Events]) => void): void

Arrow functions cannot be passed as handlers.
The handler’s payload is the union of every event’s payload, not the payload of the event being subscribed to.
Event names widen to string, so typos are accepted.
Nothing; this is the recommended signature.

You add 'on-hold' to OrderStatus. Which lookup table fails to compile until it gets an 'on-hold' entry?

const LABELS = { … } satisfies Record<OrderStatus, string>
const LABELS: Partial<Record<OrderStatus, string>> = { … }
const LABELS: Record<string, string> = { … }
const LABELS = { … } as Record<OrderStatus, string>

5 questions unanswered