Final assessment

Twelve questions across the whole course. Score 70% or more to pass and unlock your certificate. You can retake it as often as you like.

Finish every lesson to earn the certificate — you can take the assessment any time.

Which pair of changes makes both cancelOrder(customer.id) and add(usdPrice, eurShipping) compile errors?

Brand OrderId and CustomerId, and type add as add<C>(a: Money<C>, b: Money<NoInfer<C>>).
Use literal unions for statuses and satisfies on config objects.
Turn on exactOptionalPropertyTypes and noUncheckedIndexedAccess.
Give add and cancelOrder const type parameters.

What is the type of summary?

TypeScript

function pick<T, K extends keyof T>(obj: T, ...keys: K[]): Pick<T, K> { /* … */ }
const summary = pick(order, 'id', 'status');

Partial<Order>
Order
Pick<Order, keyof Order>
Pick<Order, 'id' | 'status'>

A customer’s saved cart in localStorage was written by last year’s version of the app, with a different shape. What is the right way to load it?

JSON.parse(saved) as Cart
Use JSON.parse(saved) directly, since it is typed any.
Parse it as unknown into a Result, and fall back to an empty cart (or migrate it) when parsing fails.
Call an assertion function during render and let the error boundary show a crash screen.

What is NoNulls<{ coupon: string | null; note?: string }>?

TypeScript

type NoNulls<T> = { [K in keyof T]: NonNullable<T[K]> };

{ coupon: string | null; note: string }
{ coupon: string; note?: string }
{ coupon: string; note: string }
never

The payments team adds 'requires-3ds' to the PaymentError union. errorMessage(e: PaymentError): string switches on e.type with no default. What happens?

errorMessage fails to compile until a 'requires-3ds' case is added.
It compiles and returns undefined for the new error.
It compiles and throws at runtime for the new error.
Nothing changes, because unions are checked only at runtime.

Which string is assignable to type OrderPath = `/orders/${number}`?

'/orders/abc'
'/orders/'
'/orders/1042/lines'
'/orders/1042'

After turning on noUncheckedIndexedAccess, which line becomes a compile error (with lines: Line[])?

for (const line of lines) console.log(line.sku);
lines.map((line) => line.sku);
const [first] = lines; console.log(first.sku);
console.log(lines.length);

charge is decorated with @retry(3) on the line above @logged. The gateway fails twice, then succeeds. How many log lines does logged print for one call?

1, because logged wraps the whole retrying call
3, because logged is applied first and retry calls the logged method on every attempt
0, because two decorators on one method cancel each other out
6, because each decorator logs once per attempt

Why can a DetailedOrder (id, status, totalCents, customer) be passed where an Order (id, status, totalCents) is expected, but not the other way round?

Every DetailedOrder has all the properties Order requires, so it belongs to the smaller set of values; an Order may lack customer.
Because DetailedOrder was declared with extends Order.
Because type names are compared alphabetically.
Because Order is an interface and interfaces only accept subtypes.

Which signature is an unchecked cast in disguise?

function sortByTotal<T extends { totalCents: number }>(orders: T[]): T[]
function call<R extends keyof Routes>(route: R): Routes[R]['output']
function indexById<T extends { id: number }>(items: T[]): Map<number, T>
function readSetting<T>(key: string): T

What does declare module 'legacy-shipping-calc'; with no body do?

It installs the matching @types package.
It generates types by reading the package’s JavaScript.
It lets the import compile, typing everything imported from the package as any.
It causes a compile error until the body is filled in.

Which file should you convert to TypeScript first in a migration?

checkoutPage.js, the entry point that imports 30 other modules
money.js, imported by 40 files and importing nothing internal
The largest file in the repository, to get it out of the way
The most recently changed file

12 questions unanswered