fastify/docs/Guides/Write-Plugin.md

Fast and low overhead web framework, for Node.js

<h1 style="text-align: center;">Fastify</h1>

# How to write a good plugin
First, thank you for deciding to write a plugin for Fastify. Fastify is a
minimal framework and plugins are its strength, so thank you.

The core principles of Fastify are performance, low overhead, and providing a
good experience to our users. When writing a plugin, it is important to keep
these principles in mind. Therefore, in this document, we will analyze what
characterizes a quality plugin.

*Need some inspiration? You can use the label ["plugin
suggestion"](https://github.com/fastify/fastify/issues?q=is%3Aissue+is%3Aopen+label%3A%22plugin+suggestion%22)
in our issue tracker!*

## Code
Fastify uses different techniques to optimize its code, many of which are
documented in our Guides. We highly recommend you read [the hitchhiker's guide
to plugins](./Plugins-Guide.md) to discover all the APIs you can use to build
your plugin and learn how to use them.

Do you have a question or need some advice? We are more than happy to help you!
Just open an issue in our [help repository](https://github.com/fastify/help).

Once you submit a plugin to our [ecosystem list](./Ecosystem.md), we will review
your code and help you improve it if necessary.

## Documentation
Documentation is extremely important. If your plugin is not well documented we
will not accept it to the ecosystem list. Lack of quality documentation makes it
more difficult for people to use your plugin, and will likely result in it going
unused.

If you want to see some good examples of how to document a plugin take a look
at:
- [`@fastify/caching`](https://github.com/fastify/fastify-caching)
- [`@fastify/compress`](https://github.com/fastify/fastify-compress)
- [`@fastify/cookie`](https://github.com/fastify/fastify-cookie)
- [`@fastify/under-pressure`](https://github.com/fastify/under-pressure)
- [`@fastify/view`](https://github.com/fastify/point-of-view)

## License
You can license your plugin as you prefer, we do not enforce any kind of
license.

We prefer the [MIT license](https://choosealicense.com/licenses/mit/) because we
think it allows more people to use the code freely. For a list of alternative
licenses see the [OSI list](https://opensource.org/licenses) or GitHub's
[choosealicense.com](https://choosealicense.com/).

## Examples
Always put an example file in your repository. Examples are very helpful for
users and give a very fast way to test your plugin. Your users will be grateful.

## Test
A plugin **must** be thoroughly tested to verify that is working properly.

A plugin without tests will not be accepted to the ecosystem list. A lack of
tests does not inspire trust nor guarantee that the code will continue to work
among different versions of its dependencies.

We do not enforce any testing library. We use [`node:test`](https://nodejs.org/api/test.html)
since it offers out-of-the-box parallel testing and code coverage, but it is up
to you to choose your library of preference.
We highly recommend you read the [Plugin Testing](./Testing.md#plugins) to
learn about how to test your plugins.

## Code Linter
It is not mandatory, but we highly recommend you use a code linter in your
plugin. It will ensure a consistent code style and help you to avoid many
errors.

We use [`standard`](https://standardjs.com/) since it works without the need to
configure it and is very easy to integrate into a test suite.

## Continuous Integration
It is not mandatory, but if you release your code as open source, it helps to
use Continuous Integration to ensure contributions do not break your plugin and
to show that the plugin works as intended. Both
[CircleCI](https://circleci.com/) and [GitHub
Actions](https://github.com/features/actions) are free for open source projects
and easy to set up.

In addition, you can enable services like [Dependabot](https://github.com/dependabot),
which will help you keep your dependencies up to date and discover if a new
release of Fastify has some issues with your plugin.

## Let's start!
Awesome, now you know everything you need to know about how to write a good
plugin for Fastify! After you have built one (or more!) let us know! We will add
it to the [ecosystem](https://github.com/fastify/fastify#ecosystem) section of
our documentation!

If you want to see some real world examples, check out:
- [`@fastify/view`](https://github.com/fastify/point-of-view) Templates
  rendering (*ejs, pug, handlebars, marko*) plugin support for Fastify.
- [`@fastify/mongodb`](https://github.com/fastify/fastify-mongodb) Fastify
  MongoDB connection plugin, with this you can share the same MongoDB connection
  pool in every part of your server.
- [`@fastify/multipart`](https://github.com/fastify/fastify-multipart) Multipart
  support for Fastify.
- [`@fastify/helmet`](https://github.com/fastify/fastify-helmet) Important
  security headers for Fastify.