TypeScript Intermediate — Series 2

Preview — 3 of 10 questions

What JavaScript does TypeScript emit for this const enum reference?

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

BStatus.Active is inlined as 0; no enum object is emitted.

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

Do both branches type-check after the instanceof test?

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

CBoth branches compile: instanceof narrows animal to Dog and then Cat.

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

How does checking kind affect the type of shape?

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;
}

AThe kind check narrows shape to the member with that literal discriminant.

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

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