Exploring the options parameter

This section is a deep look at the optional options parameter and how we can develop reusable plugins. A plugin declaration function isn’t anything more than a factory function that, instead of returning some new entity as factory functions usually do, adds behaviors to a Fastify instance. If we look at it like this, we can think about the options parameter as our constructor’s arguments.

Firstly, let’s recall the plugin declaration function signature:

async function myPlugin(fastify, [options])

How can we pass our custom arguments to the options parameter? It turns out that the register method has a second parameter too. So, the object we use as the argument will be passed by Fastify as Plugin’s options parameter:

app.register(myPlugin, { first: 'option'})

Now, inside the myPlugin function, we can access this value simply using options.first.

It is worth mentioning that Fastify reserves three specific...