Advanced OOP Patterns — Series 3

Preview — 3 of 10 questions

javascript

abstract class Base {
  abstract run(): void;
}

type Ctor<T> = new (...args: any[]) => T;

const c: Ctor<Base> = Base;

ACompiles fine — a class value always matches a construct signature for its instance type

BCompiles fine, but new c() throws at runtime

CCompile-time error — an abstract class cannot be assigned to a concrete construct signature; you need abstract new (...args: any[]) => T

DCompile-time error — Ctor<T> is invalid because new signatures may not be generic

javascript

class Point {
  constructor(public x: number, readonly y: string) {}
}

type I = InstanceType<typeof Point>;
type C = ConstructorParameters<typeof Point>;

AI is Point, C is [x: number, y: string]

BI is typeof Point, C is { x: number; y: string }

CI is Point, C is [number] — only public parameters are counted

DCompile-time error — these utilities require an explicitly declared constructor type, not a class

javascript

// noImplicitOverride: true
class Base {
  greet(): string { return "hi"; }
}

class Child extends Base {
  override greeet(): string { return "yo"; }
}

ACompiles fine — override is documentation and is not checked

BCompiles fine, and greet() on a Child returns "yo"

CCompile-time error — override may only be applied to a method that has no body

DCompile-time error — greeet is marked override but no such member exists on the base class

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