Checkpoint: Type-Level Programming

Question 1 of 5

type Flags<T> = { [K in keyof T as T[K] extends boolean ? K : never]: T[K] };
type F = Flags<{ giftWrap: boolean; note: string; express: boolean }>;

• { giftWrap: boolean; note: never; express: boolean }
• 'giftWrap' | 'express'
• { note: string }
• { giftWrap: boolean; express: boolean }

Question 2 of 5

type IdOf<T> = T extends { id: infer I } ? I : never;
type X = IdOf<{ id: number } | { id: string } | { name: string }>;

• number | string
• never, because the union as a whole has no common id
• number, from the first member only
• unknown, because infer cannot capture from a union

Question 3 of 5

• 'onshipped' | 'oncancelled'
• `on${string}`
• 'onShipped' | 'onCancelled'
• 'onShipped | Cancelled'

Question 4 of 5

type Draft<T> = { -readonly [K in keyof T]?: T[K] };

• { readonly id: number; status: string }, unchanged
• { id: number; status: string }
• { readonly id?: number; status?: string }
• { id?: number; status?: string }

Question 5 of 5

type ElementOf<T> = T extends (infer E)[] ? E : T;

• string
• readonly string[]
• never
• unknown