What is a composition root in the context of dependency injection?

Question

I am exploring dependency injection and the term composition root is used all over the place. So what is it?

Answer 1

Mark Seemann wrote a great article about Composition Root design pattern.

essential points from this article are:

A Composition Root is a (preferably) unique location in an application where modules are composed together.

Only applications should have Composition Roots. Libraries and frameworks shouldn't.

A DI Container should only be referenced from the Composition Root. All other modules should have no reference to the container.

http://blog.ploeh.dk/2011/07/28/CompositionRoot/

I wrote my own JavaScript Dependency Injection Framework called Di-Ninja with these principles in mind

https://github.com/di-ninja/di-ninja

As I know, is the only one in javascript that implement the Composition-Root design pattern and it's documentation could be another good example to demonstrate how it works.

It work with NodeJS, browser (with Webpack or UMD/AMD), and React-Native.