Improving developer experience for Fastify + Envalid users

[simenandre]

Hello 👋

Thank you for this library!

I have a few suggestions to make this a helpful library for more users. The way we use envalid along Fastify is where we decorate a variable called config (or similar) with the result of cleanEnv. This produces a easy way to get configuration value. Typically we have one configuration file (config.ts). It looks something like this:

import { cleanEnv, str } from 'envalid';

// In reality, we also add a custom middleware here 👇
export const config = cleanEnv(process.env, {
	HELLO_WORLD: str({ devDefault: 'something' }),
});

export type Config = typeof config;

Along with this, we add the Config type to a global type definition file as such:

import { Config } from './config.js';

declare module 'fastify' {
  interface FastifyRequest {
    config: Config;
  }
}

And lastly, in our server.ts file, you'll typically see:

// ...
server.decorateRequest('config', config);
// ...

I haven't tried, but I think we can wrap some (or all) of this in a Fastify plugin for a better developer experience. I imagine something like this:

In our config.ts file:

import { fastifyEnv } from 'fastify-envalid';
import { str } from 'envalid';

export const config = fastifyEnv({
	HELLO_WORLD: str({ devDefault: 'something' }),
});

I imagine fastifyEnv (or another name) could come prebuilt with custom middleware and wired with environment variables (potentially changed with the fastify.register function).

Moving on to the server.ts file:

import { fastifyEnvalid } from 'fastify-envalid';
import { config } from './config.js';

// ...
server.register(fastifyEnvalid, { config });
// ...

I think we can wire the types somehow here so they're available just by registering the plugin. Not sure, I haven't played around with it yet. Regardless, what do you think?

[simenandre]

Not to mention that the plugin can easily fix this: af/envalid#141