While studying the source code in depth, I found it somewhat difficult to follow how the various pieces are connected. I didn’t expect it to be trivial and overall it's reasonably comprehensible. But I believe we can improve clarity and structure with some targeted refactoring.

Scope

This PR is focusing on the main file fastify.js that currently spans nearly 1,000 lines. It contains a mix of responsibilities: options processing, instance methods, helper functions, and bootstrapping logic. This makes onboarding and maintenance harder than it needs to be.

Refactoring Proposal

I offer to extract most of the setup logic into dedicated files under a new lib/setup/ directory:

(Update: use kebab-case)

lib/
└── setup/
    ├── setup-avvio.js
    ├── setup-client-error-handler.js
    ├── setup-inject.js
    └── setup-ready.js

Additionally, I'm moving some functions into files where their concerns are better aligned.
For example:

• defaultRoute and onBadUrl → lib/route.js
• addHook → lib/hooks.js

Readability

To help navigate within fastify.js, I offer to add section markers to delineate the main phases of instance creation:

// === OPTIONS PREPROCESSING ===========================================
//
// Normalize and validate the user-supplied `options` object,
// then fill in default values.

...
// === INSTANCE OBJECT =================================================
//
// Construct the main `fastify` object, including the public API.
//

And others (INSTANCE SETUP PHASE, INSTANCE METHODS, INTERNAL HELPERS...).

While such comments aren't a substitute for well-structured code, they provide helpful guidance given that we're still working within a single large factory function rather than splitting it into smaller modules.