Checkpoint: Types as a Design Tool

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

A reviewer worries that the card number reaches saveAddress. What is true?

TypeScript

type Address = { line1: string; city: string };
function saveAddress(a: Address) { /* sends a to the address service */ }
const form = { line1: '12 Nile St', city: 'Cairo', cardNumber: '4242 4242 4242 4242' };
saveAddress(form);

It fails to compile, because cardNumber is an excess property.
TypeScript removes cardNumber before the call, because Address does not declare it.
It compiles only if you write form as Address.
It compiles, and the whole object, card number included, is passed at runtime; copy the fields you need explicitly.

quantityInStock is null while stock is being checked and 0 when an item is sold out. Which change makes this label correct?

TypeScript

function stockLabel(quantityInStock: number | null): string {
  if (!quantityInStock) return 'Checking stock…';
  return quantityInStock === 0 ? 'Sold out' : `${quantityInStock} left`;
}

Replace !quantityInStock with quantityInStock === null.
Add if (typeof quantityInStock === 'number') after the first line.
Write quantityInStock! in the comparison.
Remove the : string return type so TypeScript infers it.

Cartwheel adds payments. A pending payment has no capture id; a captured one always has captureId; a refunded one always has captureId, refundedAt and refundCents. Which type makes the impossible combinations fail to compile?

{ status: 'pending' | 'captured' | 'refunded'; captureId?: string; refundedAt?: string; refundCents?: number }
{ status: string; details: Record<string, unknown> }
A union of three object types, each with a literal status and exactly the fields that state has, all required.
Partial<{ status: string; captureId: string; refundedAt: string; refundCents: number }>

What is ShippingMethod?

TypeScript

const SHIPPING_FEES = { standard: 499, express: 1299 } satisfies Record<string, number>;
type ShippingMethod = keyof typeof SHIPPING_FEES;

string, because the object was checked against Record<string, number>
'standard' | 'express'
number, the type of the values
'standard' | 'express' | string

A switch (order.status) has case 'canceled': with one L, and it never matches. Which change turns that typo into a compile error?

Add a default: branch that logs unknown statuses.
Annotate order as any so TypeScript checks each case.
Rewrite the switch as an if/else chain.
Type status as the union of the five literal statuses instead of string.

5 questions unanswered