Middlewares

With this adapter you can create procedure middlewares, those middlewares then can be added to a router class method by the @Middlewares() decorator.
The tRPC concept of middlewares is comparable to how Guards and Middlewares work in NestJS.

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If you are not sure about the basic concepts of NestJS or tRPC middlewares, you can dive into those concepts in their official documentation.

What are NestJS Middlewares→What are tRPC Middlewares→

Creating a Middleware

You can implement a middleware in a class with an @Injectable() decorator. The class should implement the TRPCMiddleware interface. Let's start by implementing a simple logger middleware.

logger.middleware.ts

import { TRPCMiddleware, TRPCMiddlewareOptions } from 'nestjs-trpc';
import { Inject, Injectable, ConsoleLogger } from '@nestjs/common';
import type { Context } from "nestjs-trpc/types";
 
@Injectable()
export class LoggedMiddleware implements TRPCMiddleware<Context> {
 
  constructror(
    @Inject(ConsoleLogger) private readonly consoleLogger: ConsoleLogger
  ) {}
 
  use(opts: TRPCMiddlewareOptions) {
    const start = Date.now();
    const { next, path, type } = opts;
    const result = await next();
 
    const durationMs = Date.now() - start;
    const meta = { path, type, durationMs }
 
    result.ok
      ? this.consoleLogger.log('OK request timing:', meta)
      : this.consoleLogger.error('Non-OK request timing', meta);
 
    return result;
  }
}

Middleware registration

Similar to NestJS providers, we need to register the middleware with Nest so that it can perform the injection and type generation. We do this by editing our module file and adding the middleware to the providers array of the @Module() decorator.

Applying middleware

To use the middleware in your procedure instead of the default publicProcedure, pass the class to the @Middlewares() method decorator.

dogs.router.ts

import { DatabaseService } from "./database.service.ts";
import { LoggedMiddleware } from "./logged.middleware.ts";
import { Router, Query, Middlewares } from 'nestjs-trpc';
import { Inject } from '@nestjs/common';
import { z } from 'zod';
 
const dogsSchema = z.object({
  name: z.string(),
  breed: z.enum(["Labrador", "Corgi", "Beagle", "Golden Retriver"])
});
 
@Router()
export class DogsRouter {
  constructor(@Inject(DatabaseService) private databaseService: DatabaseService){}
 
  @Middlewares(LoggedMiddleware)
  @Query({ output: z.array(dogSchema) })
  async findAll(): string {
    const dogs = await this.databaseService.dogs.findMany();
    return dogs;
  }
}

Modified Context

With middlewares you can change and inject certain parameters in the context to be available to the procedure, this can be done by using the next() method available from the opts parameter.

Generated Context

When you modify the base context with a middleware, a new type will be generated with the middleware name and a Context suffix, for example if you have an AuthMiddleware, the generated type would be named AuthMiddlewareContext.

Then you can import those types and use throughout your procedures from nestjs-trpc/types, for example:

user.router.ts

import type { AuthMiddlewareContext } from 'nestjs-trpc/types';
import { UserService } from "./user.service.ts";
import { AuthMiddleware } from "./auth.middleware.ts";
import { Router, Query, Middlewares } from 'nestjs-trpc';
import { Inject } from '@nestjs/common';
import { z } from 'zod';
 
const userSchema = z.object({
  name: z.string(),
  email: z.string(),
  avatar: z.string()
});
 
@Router()
export class UserRouter {
  constructor(@Inject(UserService) private userService: UserService){}
 
  @Middlewares(AuthMiddleware)
  @Query({ output: userSchema })
  async getUserProfile(@Context() ctx: AuthMiddlewareContext): string {
    const { userId } = ctx.auth;
    return await this.userService.getUserById(userId);
  }
}

Dependency injection

Middlewares fully supports Dependency Injection. Just as with NestJS middlewares and guards, they are able to inject dependencies that are available within the same module. As usual, this is done through the constructor.

Routers Context