Checkpoint: Generics in Depth

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

Which signature lets firstWhere(orders, (o) => o.status === 'shipped') return Order | undefined, and firstWhere(products, …) return Product | undefined, with o typed in the callback?

function firstWhere<T>(items: T[], predicate: (item: T) => boolean): T | undefined
function firstWhere(items: unknown[], predicate: (item: unknown) => boolean): unknown
function firstWhere<T>(items: unknown[], predicate: (item: T) => boolean): T | undefined
function firstWhere(items: any[], predicate: (item: any) => boolean): any

What is the type of t.value?

TypeScript

function tag<T>(value: T) {
  return { value };
}
const t = tag('shipped');

'shipped'
unknown
any
string

setDefault(['standard', 'express'], 'overnight') should be rejected because 'overnight' is not one of the options. Which signature does that?

function setDefault<const T>(options: T[], fallback: T)
function setDefault<T extends string>(options: T[], fallback: T)
function setDefault<T extends string>(options: T[], fallback: NoInfer<T>)
function setDefault<T>(options: T[], fallback: NoInfer<T>)

Callers of Cartwheel’s client write api.get<Order>('/orders/1042') everywhere, and last week someone wrote api.get<Order[]>('/orders/1042'). What is the best fix?

Key the client by a route map type, so the response type is looked up from the route and callers pass no type argument.
Add an ESLint rule that requires a type argument on every get call.
Change the default type parameter of get to any.
Make get return unknown and let each caller cast with as.

function useQuery<TData, TKey extends string>(key: TKey, fetcher: () => Promise<TData>) is called as useQuery<Order>('order-1042', loadOrder) and fails with “Expected 2 type arguments, but got 1”. What is the best fix at the call site?

Write useQuery<Order, string>(…) everywhere.
Remove <Order> and let both type parameters be inferred, TData from loadOrder’s return type.
Wrap the key in NoInfer.
Cast the call result to any.

5 questions unanswered