TypeScript Intermediate — Series 2

Preview — 3 of 10 questions

javascript

const enum Status {
  Active,
  Inactive,
}
let s = Status.Active;

Aconst enum members can't be accessed at all outside the file where they're declared

Bconst enum is inlined at compile time — the emitted JavaScript replaces Status.Active with its literal value 0, and no Status object exists at runtime

Cconst enum requires every member to have an explicit value

Dconst enum behaves exactly like a regular enum — const is purely cosmetic and has no compilation effect

javascript

class Dog { bark() { return "Woof"; } }
class Cat { meow() { return "Meow"; } }
function speak(animal: Dog | Cat) {
  if (animal instanceof Dog) {
    return animal.bark();
  }
  return animal.meow();
}

ACompile-time error on animal.bark() — animal is still typed Dog | Cat inside the if block

BCompile-time error on animal.meow() — Dog was already checked first

CCompiles fine — instanceof Dog narrows animal to Dog inside the if branch, and to Cat in the remaining branch, so both method calls type-check without any assertion

DCompiles fine, but only after adding an explicit as Dog / as Cat assertion

javascript

type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number };

function area(shape: Shape): number {
  if (shape.kind === "circle") {
    return Math.PI * shape.radius ** 2;
  }
  return shape.side ** 2;
}

AComparing the shared literal kind field narrows shape to the specific union member whose kind matches — TypeScript uses kind as a discriminant to know which shape it's dealing with

Bradius and side are treated as optional properties present on every member of the union

CTypeScript widens shape to any inside conditional branches, so any property access is allowed

DThis is a compile-time error — shape.radius isn't accessible without an explicit type assertion

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