How to properly decorate request in TypeScript

[awais-shafiq]

I'm trying this code and it fails to compile and throws an error.

	server.decorateRequest('user', null);

	server.addHook('preHandler', (request, _reply, done) => {
		request.user = { id: 10001 };
		done();
	});

	server.get('/', (request, reply) => {
		console.log(request.user);
		reply.code(200).send({ success: true });
	});

Compile Error

Property 'user' does not exist on type 'FastifyRequest<RouteGenericInterface, RawServerDefault, IncomingMessage, FastifySchema, FastifyTypeProviderDefault, unknown, FastifyBaseLogger, ResolveFastifyRequestType<...>>'

Fastify version: 4.23.2, 4.24.0, 4.24.2
Node Version: 18.17.0
Typescript version: 5.2.2

tsconfig.json

{
  "compilerOptions": {
    
    "target": "ES2017",                         
    "module": "commonjs",                       
    "rootDir": "./src",                         
    "allowJs": false,                           
    "outDir": "./dist",                         
    "noEmitOnError": true,                      
    "preserveConstEnums": true,                 
    "esModuleInterop": true,                    
    "forceConsistentCasingInFileNames": true,   
    "strict": true,                             
    "noImplicitAny": true,                      
    "strictNullChecks": true,                   
    "strictFunctionTypes": true,                
    "strictPropertyInitialization": true,       
    "useUnknownInCatchVariables": true,         
    "alwaysStrict": true,                       
    "noUnusedLocals": true,                     
    "noUnusedParameters": true,                 
    "noImplicitReturns": true,                  
    "noUncheckedIndexedAccess": true,           
    "allowUnreachableCode": true,               
    "skipLibCheck": true,                       
    "experimentalDecorators": true
  },
  "include": ["./src/**/*.ts"],
  "exclude": ["node_modules"]
}

[metcoder95]

Hey!

Remember to do interface augmentation when doing decorations to Fastify using TS, this is an example:

declare module 'fastify' {
	interface FastifyRequest {
		user: {
			id: number
		}
	}
}

[awais-shafiq]

In my /src/ folder I created custom.d.ts file and augmented the interface as following. I'm still getting same error.

declare module 'fastify' {
	interface FastifyRequest {
		user: {
			id: number;
		};
	}
}

[metcoder95]

You need to do the augmentation on the same place as the application code, otherwise, it rewrites the module type definition, leading to errors. A working example might be:

import { fastify } from 'fastify';

declare module 'fastify' {
  interface FastifyRequest {
    user: {
      id: number;
    };
  }
}

const server = fastify();

server.decorateRequest('user', null);

server.addHook('preHandler', (request, _reply, done) => {
  request.user = { id: 10001 };
  done();
});

server.get('/', (request, reply) => {
  console.log(request.user);
  reply.code(200).send({ success: true });
});

[awais-shafiq]

Attached my tsconfig to original question.

[awais-shafiq]

@metcoder95 Thanks!
Your solution works.

But augmenting interface in custom.d.ts file also worked.
I was using ts-node to run my app. It was not loading augmented interface. The solution was passing --files flag to ts-node command

"scripts": {
    "build": "tsc ",
    "start": "node ./dist/server.js",
    "dev": "ts-node  --files ./src/server.ts "
 },

I was able to figure this out when I directly tried building and it did not give any error at compile time.

Again Thanks for your help.

[truongnat]

create file fastify.d.ts in src/@types

declare module "fastify" {
  export interface FastifyInstance<
    HttpServer = http.Server,
    HttpRequest = http.IncomingMessage,
    HttpResponse = http.ServerResponse
  > {
    blipp(): void; // declare here
  }
}