Guide: Dependency Injection via the Fastify Plugin System

[matthyk]

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Issue

I am currently working on a talk about the capability to do dependency injection through Fastify's plugin system without using an IoC container, as you might know from the Java ecosystem or the popular Node.js framework Nest.js. Would there be any interest in a guide explaining how to use this specifically in Fastify (with and without TypeScript) and how it can be used to mock dependencies during testing?
If so, I would be happy to write the guide over the next few weeks while I have to prepare the talk anyway.

[mcollina]

Definitely!

[jsumners]

I assert that the fastify instance is very similar to an IoC container.

[matthyk]

I have the feeling that there is still a lot of confusion when testing Fastify applications. I think the main reason for this is Fastify's plugin system and the resulting encapsulation. Because there are different contexts but you can only access the root context directly, the classic mocking/spying is not possible as you might know it from other frameworks. I would therefore suggest taking a different thematic approach to the guide that I originally proposed, and rather address the problems discussed above and discuss possible solutions. There are already plugins written by the community that address these problems, which solve the problem in different ways. You could therefore see this guide as a collection of different solutions and test strategies.For example:

• https://github.com/Eomm/fastify-explorer
• https://github.com/matthyk/fastify-override

Here are some discussions/StackOverflow questions around this topic:

• Mocking a plugin #4927
• Overriding decorators for testing proposes #5324
• https://stackoverflow.com/questions/59965807/how-to-mock-fastify-plugin

What are your thoughts on this?