How do you create custom decorators?

Question

IT Interview Practice · Accepted Answer

NestJS lets you create **custom decorators** to encapsulate reusable logic — most commonly **parameter decorators** (to extract data from the request) and **metadata decorators** (to attach data read by guards/interceptors). They keep code DRY and expressive.

## Custom parameter decorator — extract request data

```ts
import { createParamDecorator, ExecutionContext } from "@nestjs/common";

// extract the authenticated user (attached to the request by an auth guard)
export const CurrentUser = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.user;        // return whatever you want injected
  },
);

// usage — clean and readable
@Get("profile")
getProfile(@CurrentUser() user: User) {     // instead of @Req() req and req.user
  return user;
}
```

`createParamDecorator` builds a parameter decorator: its function receives the `ExecutionContext`, extracts data, and returns the value to inject. This replaces repetitive `@Req() req` + `req.user` access with a clean, reusable `@CurrentUser()`.

## Parameter decorator with arguments

```ts
// extract a specific property: @CurrentUser("email")
export const CurrentUser = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const user = ctx.switchToHttp().getRequest().user;
    return data ? user?.[data] : user;   // `data` is the argument passed to the decorator
  },
);
// @CurrentUser("email") email: string  → injects just user.email
```

## Custom metadata decorator (for guards/interceptors to read)

```ts
import { SetMetadata } from "@nestjs/common";

// a @Roles() decorator that ATTACHES metadata to a route
export const Roles = (...roles: string[]) => SetMetadata("roles", roles);

// usage
@Roles("admin", "editor")
@Get("users")
listUsers() {}
```

```ts
// a guard reads the metadata via Reflector
@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private reflector: Reflector) {}
  canActivate(ctx: ExecutionContext): boolean {
    const roles = this.reflector.get<string[]>("roles", ctx.getHandler());   // read it
    const { user } = ctx.switchToHttp().getRequest();
    return !roles || roles.some(r => user.roles?.includes(r));
  }
}
```

The `@Roles()` + `RolesGuard` combination — a metadata decorator that a guard reads via `Reflector` — is the canonical pattern for role-based authorization.

## Composing decorators

```ts
import { applyDecorators } from "@nestjs/common";
// bundle multiple decorators into one reusable decorator
export function Auth(...roles: string[]) {
  return applyDecorators(SetMetadata("roles", roles), UseGuards(AuthGuard, RolesGuard));
}
// usage: @Auth("admin")  — applies metadata + guards in one decorator
```

## Why it matters

Creating custom decorators is an important NestJS technique for keeping code DRY, expressive, and maintainable. **Custom parameter decorators** (`createParamDecorator`) let you encapsulate common request-data extraction (like `@CurrentUser()`) into clean, reusable annotations — far better than repeating `@Req() req` and digging into the request object in every handler. **Custom metadata decorators** (`SetMetadata`) are the foundation of NestJS's authorization patterns: the standard `@Roles()` + `RolesGuard` (read via `Reflector`) approach for role-based access control depends entirely on understanding metadata decorators.

And **composing decorators** (`applyDecorators`) lets you bundle related decorators (e.g. an `@Auth()` that applies guards and metadata together) for concise, consistent application.

Understanding how to create these is what lets you build expressive, idiomatic NestJS APIs and implement common patterns (current-user injection, RBAC) the framework-intended way.

It elevates you from merely using built-in decorators to extending the framework's declarative style for your own application's needs — a valuable, frequently-applied skill in real NestJS projects.

Custom parameter decorator — extract request data

Parameter decorator with arguments

Custom metadata decorator (for guards/interceptors to read)

Composing decorators

Why it matters

Related questions