Type-Level Generics — Series 2

Preview — 3 of 10 questions

javascript

type UserId = string & { readonly __brand: 'UserId' };
type PostId = string & { readonly __brand: 'PostId' };

function createUserId(id: string): UserId { return id as UserId; }
function getUser(id: UserId) { /* ... */ }

const uid = createUserId('u1');
const pid = 'p1' as PostId;

getUser(uid);
getUser(pid);

AgetUser(uid) compiles fine; getUser(pid) is a compile-time error, because PostId's brand ('PostId') doesn't match the 'UserId' brand getUser requires — even though both types are structurally "just a string" underneath

BBoth calls compile fine — TypeScript is always fully structurally typed, so the extra brand properties are ignored

CBoth calls throw a runtime TypeError

DNeither compiles — intersecting string with an object literal type like { readonly __brand: 'UserId' } is invalid syntax

javascript

function firstElement<const T extends readonly unknown[]>(arr: T): T[0] {
  return arr[0];
}
const result = firstElement(['a', 'b', 'c']);

Aresult has type string — the const modifier has no effect on generic inference

Bresult has type "a" — the const type-parameter modifier (TS 5.0) tells the compiler to infer the narrowest possible (literal) type for T, instead of widening the array to string[]

CThis is a compile-time error — const cannot be used as a type parameter modifier

Dresult has type readonly ["a", "b", "c"] — the whole tuple, not just its first element

What does the asserts val is string return-type annotation do?

javascript

function assertIsString(val: unknown): asserts val is string {
  if (typeof val !== 'string') throw new Error('Not a string');
}
function process(val: unknown) {
  assertIsString(val);
  return val.toUpperCase();
}

AIt has no effect on type narrowing — val is still unknown after the call, so this code shouldn't actually compile

BIt automatically converts val to an actual string value at runtime

CIf assertIsString returns normally (i.e., doesn't throw), TypeScript narrows val to string for the rest of the enclosing scope — so val.toUpperCase() compiles without needing an explicit if type-guard block

DIt only works when the call is placed inside an if statement, not as a standalone statement

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