this: Object is possibly 'undefined

[bkawk]

Prerequisites

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

Fastify version

3.0.0

Plugin version

3.0.0

Node.js version

14.15.4

Operating system

macOS

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

11.5.2

Description

In a TypeScript environment when trying to access this as shown in the docs, inside the route it's always possibly undefined. If I ensure it defined I get `Property 'mongo' does not exist on type 'never'

Steps to Reproduce

I've set up the plugin with

import fp from 'fastify-plugin';
import mongodb from 'fastify-mongodb';

export default fp(async (fastify) => {
  fastify.register(mongodb, {
    url: 'mongodb+srv://dbuser:mypassword@cluster0.otigz.mongodb.net/myapp?retryWrites=true&w=majority',
  });
});

then in the handler, I have

const postJoinHandler = async (
  request: any,
  reply: any
): Promise<{ id: string; name: string }> => {
  try {
    const { username, password } = request.body;
    
    const test = await this.mongo.db.collection('users');
    test.insertOne({
      username,
      password,
    });

    return reply.code(201).send(username);
  } catch (error) {
    request.log.error(error);
    return reply.send(400);
  }
};

if I try to make sure this is not denied with...

    const t = this;
    if (t) {
      const test = t.mongo.client.db('mydb').collection('users');
      test.insertOne({
        username,
        password,
      });
      console.log(test);
    }

I get `Property 'mongo' does not exist on type 'never'

in the router I have

import { FastifyPluginAsync } from 'fastify';
import { postJoinSchema, postLoginSchema } from '../schemas/auth';

const auth: FastifyPluginAsync = async (fastify): Promise<void> => {
  fastify.post('/auth/join', postJoinSchema);
  fastify.post('/auth/login', postLoginSchema);
};

export default auth;

Full example here https://github.com/bkawk/fastify-swagger/blob/mongodb/src/handlers/auth.ts#L9-L18

Expected Behavior

I expect to be able to write to the database by following the examples in the readme

[climba03003]

this is expected to be undefined when you use arrow function.

It is the nature of JavaScript. arrow function can never change the context of this.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

Does not have its own bindings to this or super, and should not be used as methods.