Prerequisites

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

Issue

I can understand a certain level of conservatism in focusing mainly on features, bug fixes, and performance.

That said, I think it’s still worth asking:
Are there things we could do to improve the structure and readability of the codebase without hurting performance?

For example:

• @jsumners has previously suggested adding more JSDoc if I remember correctly, which could help with clarity.
• As noted in refactor: clean fastify mainfile #6107, some functions have grown quite large and become messy as features and fixes accumulate.
• As an example, in fastify-autoload#401, I did a significant refactor to reduce cognitive complexity in the tree traversal logic.
• Should we consider modernizing some syntax (e.g. class instead of function constructors, using more spread operators, etc.)?