Decorator & Adapter Patterns (Advanced)

Decorators: Extending Without Editing

A decorator is a function that receives the decorated target and can wrap or annotate it. TypeScript 5 supports the ECMAScript standard decorators by default; "experimentalDecorators": true opts into the older legacy semantics that Angular and older NestJS code rely on:

function logged(target: Function, context: ClassMethodDecoratorContext) {
  const name = String(context.name);
  return function (this: unknown, ...args: unknown[]) {
    console.log(`${name}(`, args, ")");
    return target.apply(this, args);
  };
}

class Svc { @logged run(x: number) { return x * 2; } }

Standard decorators receive a context object with name and kind; only the legacy path supports emitDecoratorMetadata, which dependency-injection frameworks use for constructor injection.

Adapter: Fitting a Foreign Interface

An adapter translates an interface you cannot change into the one your code wants — third-party SDKs, legacy services, or a vendor payload with different field names:

interface Payee { charge(cents: number): Promise<{ ok: boolean }> }

class StripeAdapter implements Payee {
  constructor(private stripe: StripeSDK) {}
  async charge(cents: number) {
    const r = await this.stripe.charges.create({
      amount: cents, currency: "usd",
    });
    return { ok: r.status === "succeeded" };
  }
}

Your domain depends on Payee; swapping Stripe for a test fake is a constructor change at the composition root. Adapters also normalize errors, so callers see one failure type instead of a vendor's exception hierarchy.

Pitfalls and Interview Angles

Decorator order. Multiple decorators on one method apply bottom-up — the closest to the method runs first when wrapping. State the order explicitly when one decorator adds auth and another logs.

this loss. Passing obj.method as a callback detaches this; use obj.method.bind(obj) or an arrow property, or your wrapper's target.apply(this, args) sees undefined.

Adapter creep. Adapters should translate, not accumulate business logic; if validation and branching pile up, the domain model is leaking.

Interview: implement an adapter live, then explain standard versus legacy decorators and which tsconfig flag flips between them.