Advanced OOP Patterns

Preview — 3 of 10 questions

What does this type represent?

javascript

type Constructor<T = {}> = new (...args: any[]) => T;

function getInstance<T>(Ctor: Constructor<T>): T {
  return new Ctor();
}

AA type alias for any function

BA generic interface for factories

CA type that represents a class constructor — something you can call with new to produce a T

DA type that only matches abstract classes

What does InstanceType extract?

javascript

class Database {
  connect(url: string): void {}
  query(sql: string): Promise<unknown[]> { return Promise.resolve([]); }
}

type DB = InstanceType<typeof Database>;
// DB is: ?

Atypeof Database — the class constructor type

B{ connect: Function; query: Function } — a plain object type

CDatabase — the instance type (same as just writing Database)

DConstructor<Database>

What does a class decorator do?

javascript

function Singleton<T extends Constructor>(Base: T): T {
  let instance: InstanceType<T>;
  return class extends Base {
    constructor(...args: any[]) {
      if (instance) return instance;
      super(...args);
      instance = this as unknown as InstanceType<T>;
    }
  } as T;
}

@Singleton
class AppConfig {
  theme = 'dark';
}

const c1 = new AppConfig();
const c2 = new AppConfig();
// c1 === c2?

Atrue — @Singleton wraps the class so all new AppConfig() calls return the same instance

Bfalse — @Singleton has no effect on class instantiation

CCompile error — decorators cannot modify constructors

Dfalse — c1 and c2 are always different objects

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