Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

3.25.0

Plugin version

2.2.0

Node.js version

16.x

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

12.0.1 Monterey

Description

With a project using fastify and fastify-request-context, the user could get confused because

declare module 'fastify-request-context' {
    interface RequestContextData {
       a: number;
    }
}

const value = request.requestContext.get('string');

and value is set to number | undefined. In the documentation it states that the value should be set to a type of number, but this is not always the case.

Steps to Reproduce

Create a new folder/nodejs project with these files.

package.json

{
  "name": "fastify-request-context-typescript",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "fastify": "^3.25.0",
    "fastify-request-context": "^2.2.0"
  },
  "devDependencies": {
    "@types/node": "^17.0.0"
  }
}

tsconfig.json

{
    "compilerOptions": {
        "strict": true // the cause of the issue
    }
}

app.ts

import fastify, { FastifyLoggerInstance } from 'fastify';
import { fastifyRequestContextPlugin } from 'fastify-request-context';

import { simpleController } from './a';

declare module 'fastify-request-context' {
    interface RequestContextData {
        logger: FastifyLoggerInstance;
    }
}

const server = fastify();

server.register((app) => {
    app.get('/', {
        handler: simpleController
    });
}, {
    prefix: '/helloworld',
})

server.register(fastifyRequestContextPlugin, {
    defaultStoreValues: {
        logger: server.log,
    },
});

a.ts

import { RouteHandler } from 'fastify';

export const simpleController: RouteHandler = async (req, reply) => {
    const logger = req.requestContext.get('logger');
    
    // logger is possibly undefined, so this is a typescript error
    logger.info('do a thing');

    reply.code(204);
};

The issue is due to this typing I think:

https://github.com/fastify/fastify-request-context/blob/master/index.d.ts#L8

Since the type is RequestContextData[K] | undefined, in strict typing mode I think that this causes logger above in a.ts to be FastifyLoggerInstance | undefined. This is not the case if you add "strictNullChecks": false to the tsconfig.json.

Expected Behavior

Could the .get method just be typed as get<K extends keyof RequestContextData>(key: K): RequestContextData[K]? This seems to work with "strictNullChecks" set to either true (implicitly by "strict": true), or with "strict": true and "strictNullChecks": false.