Mapped & Conditional Types — Series 3

Preview — 3 of 10 questions

What is config's type, and does the assignment compile?

javascript

const config = { mode: "dark", retries: 3 } as const;

config.retries = 5;

A{ mode: string; retries: number } — the assignment compiles

B{ readonly mode: string; readonly retries: number } — the assignment fails

C{ readonly mode: "dark"; readonly retries: 3 } — the assignment fails, because every property is readonly

D{ mode: "dark"; retries: 3 } — the assignment fails because 5 isn't the literal 3

javascript

function describe(value: string | null) {
  if (typeof value === "object") {
    return value.toFixed(2);
  }
  return value.toUpperCase();
}

ACompile-time error — inside the if, value narrows to null, which has no toFixed

BCompiles fine — typeof value === "object" excludes null, so the branch is unreachable

CCompiles fine, and throws at runtime only when value is null

DCompile-time error on the else path — value is still string | null there

javascript

class HttpError extends Error {
  constructor(public status: number) {
    super(`HTTP ${status}`);
  }
}

function report(e: Error) {
  if (e instanceof HttpError) {
    return e.status;
  }
  return e.message;
}

ACompile-time error — e is declared Error, so it can never narrow to a subclass

BCompiles fine — instanceof narrows e to HttpError in the branch, and back to Error afterwards

CCompiles fine, but e.status is number | undefined because the check is only a runtime guard

DCompile-time error on e.message — after the if, e is narrowed to Error & not HttpError, which has no members

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