Conditional Types & Infer — Series 3

Preview — 3 of 10 questions

javascript

type IsStr<T> = T extends string ? "y" : "n";

type A = IsStr<unknown>;
type B = IsStr<any>;
type C = IsStr<never>;

AA is "y", B is "y", C is "n"

BA is "n", B is "y" | "n", C is never

CAll three are "n"

DA is "n", B is "y", C is "n"

javascript

function label<T extends string | number>(value: T): T extends string ? "s" : "n" {
  if (typeof value === "string") {
    return "s";
  }
  return "n";
}

ACompiles fine — narrowing value resolves the conditional in each branch

BCompiles fine — a conditional return type is checked only at call sites

CCompile-time error — a function's return type may not be a conditional type

DCompile-time error on both return statements — while T is unresolved the conditional stays deferred, and no concrete type is assignable to it

javascript

type SplitFirst<S extends string> = S extends `${infer A}-${infer B}` ? [A, B] : never;
type SplitChar<S extends string> = S extends `${infer A}${infer B}` ? [A, B] : never;

type X = SplitFirst<"a-b-c">;
type Y = SplitChar<"abc">;

AX is ["a", "b-c"], Y is ["a", "bc"]

BX is ["a-b", "c"], Y is ["ab", "c"]

CX is ["a", "b"], Y is ["a", "b"] — the trailing text is discarded

DBoth are never — a template literal pattern must match the whole string exactly with no ambiguity

Answer all 10 questions (7 more), see explanations for every answer, and track your score.