Advanced Type Engineering — Series 2

Preview — 3 of 10 questions

javascript

interface ApiResponse {
  data: string;
  error: string;
  status: number;
}
type ResponseValues = ApiResponse['data' | 'error'];

AResponseValues is a tuple [string, string], pairing both property types together

BCompile-time error — indexed access types only accept a single key, never a union of keys

CResponseValues is string — indexing with a union of keys retrieves the union of the corresponding property types (string | string), which simplifies to plain string

DResponseValues is ApiResponse, unchanged

javascript

type DeepElement<T> = T extends (infer U)[] ? DeepElement<U> : T;
type A = DeepElement<number[][][]>;
type B = DeepElement<string>;

AA is number[][], and B is string

BCompile-time error — a recursive type alias can't use infer inside its own definition

CA is number[][][], unchanged, and B is never

DA is number, and B is string — the recursive conditional type keeps unwrapping one array layer at a time until it reaches a type that isn't an array at all

javascript

interface User {
  id: number;
  name: string;
  password: string;
}
type PublicUser = { [K in keyof User as K extends 'password' ? never : K]: User[K] };
const u: PublicUser = { id: 1, name: 'Ana' };

APublicUser drops the password key entirely — remapping a key to never inside the mapped type's as clause causes that property to be omitted from the resulting type, so u (correctly missing password) compiles fine

BCompile-time error — mapping a key to never inside an as clause isn't valid syntax

CPublicUser retains all of User's properties, including password

DPublicUser keeps the password key, but its value type becomes never

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