What are decorators and how are they used?

Question

Accepted Answer

A **decorator** is a special declaration (`@name`) that adds behavior or metadata to a class, method, property, or parameter. It's a function the runtime calls with the decorated target, letting you wrap or annotate it declaratively.

```ts
// A simple method decorator that logs calls
function log(target: any, key: string, desc: PropertyDescriptor) {
  const original = desc.value;
  desc.value = function (...args: any[]) {     // wrap the original method
    console.log(`calling ${key} with`, args);
    return original.apply(this, args);
  };
}

class Service {
  @log
  fetch(id: number) { return { id }; }
}
new Service().fetch(1); // logs "calling fetch with [1]" then runs
```

The decorator received the method descriptor and replaced it with a wrapper — cross-cutting behavior (logging, caching, timing) without touching the method body.

## Where you see them most: frameworks

```ts
@Controller("users")          // NestJS — declares a route controller
export class UsersController {
  @Get(":id")                 // maps GET /users/:id
  findOne(@Param("id") id: string) { return id; } // param decorator injects the route param
}

@Entity()                     // TypeORM — maps a class to a DB table
class User { @Column() name: string; }
```

NestJS, Angular, TypeORM, and class-validator are built around decorators for DI, routing, ORM mapping, and validation.

## Two flavors (important)

```json
{ "experimentalDecorators": true, "emitDecoratorMetadata": true }
```

The **legacy (experimental)** decorators above are what Nest/Angular/TypeORM rely on today. There's also the newer **TC39 Stage-3 decorators** (TS 5+) with a different signature, which are the future standard — but most existing frameworks still use the legacy form.

## Why it matters

Decorators enable clean, declarative meta-programming — the backbone of server frameworks and ORMs.

Knowing both the mechanism (a function wrapping a target) and the legacy-vs-standard split is essential for working in the NestJS/Angular ecosystem.