Inheritance & Polymorphism — Series 3

Preview — 3 of 10 questions

javascript

interface Box { a: string }
interface Box { b: number }

type Crate = { a: string };
type Crate = { b: number };

ABoth pairs are errors — a name may be declared only once

BThe interface pair merges into { a: string; b: number }; the type pair is a duplicate-identifier error

CBoth pairs merge — interface and type behave identically for object shapes

DThe interface pair is an error; the type pair merges into an intersection

javascript

class Bag {
  [key: string]: number;

  count = 0;
  label = "bag";
}

ACompiles fine — index signatures on classes apply only to dynamically added keys

BCompiles fine — declared members are exempt from a class's index signature

CCompile-time error on count — a class may not declare both an index signature and named fields

DCompile-time error on label — its type string isn't assignable to the index signature's value type number

javascript

class Result<T = string> {
  constructor(public value: T) {}
}

const a = new Result("ok");
const b = new Result(42);
const c: Result = new Result("ok");

Aa is Result<string>, b is Result<number>, c is Result<string>

BAll three are Result<string> — the default overrides inference

Cb is a compile-time error — the default type parameter pins T to string

Dc is a compile-time error — a generic class always requires an explicit type argument

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