Custom Param Decorators

Create your own parameter decorators — identical to NestJS's createParamDecorator. This lets you extract any value from the Elysia context and inject it directly into controller method parameters.

createParamDecorator

import { createParamDecorator } from 'next-js-backend';
import type { Context } from 'elysia';

export const CurrentUser = createParamDecorator(
  (data: string | undefined, ctx: Context) => {
    const user = ctx.user; // set by AuthGuard
    return data ? user?.[data] : user;
  },
);

Generic Typing

createParamDecorator is fully typed:

createParamDecorator<TData, TOutput>(
  factory: (data: TData | undefined, ctx: Context) => TOutput | Promise<TOutput>
): (data?: TData) => ParameterDecorator

Usage in Controllers

import { Controller, Get } from 'next-js-backend';
import { CurrentUser } from './decorators/current-user.decorator';

@Controller('/users')
export class UserController {
  // Injects entire user object
  @Get('/me')
  getMe(@CurrentUser() user: User) {
    return user;
  }

  // Injects a specific field from user
  @Get('/email')
  getEmail(@CurrentUser('email') email: string) {
    return { email };
  }
}

More Examples

Extract Client IP

export const ClientIp = createParamDecorator(
  (_data: unknown, ctx: Context) =>
    ctx.request.headers.get('x-forwarded-for') ??
    ctx.request.headers.get('host') ??
    'unknown'
);

Extract Bearer Token

export const BearerToken = createParamDecorator(
  (_data: unknown, ctx: Context) => {
    const auth = ctx.request.headers.get('authorization') ?? '';
    return auth.startsWith('Bearer ') ? auth.slice(7) : null;
  }
);

Async Factory

The factory can be async — it will be awaited before the controller method runs:

export const SessionUser = createParamDecorator(
  async (_data: unknown, ctx: Context) => {
    const token = ctx.request.headers.get('authorization');
    if (!token) return null;
    return await authService.validateToken(token);
  }
);

How It Works

createParamDecorator stores the factory function in route metadata using reflect-metadata. When a request arrives, ElysiaFactory calls factory(customData, ctx) for each CUSTOM param before invoking the controller method.