Summary
So to recap, we have seen in this chapter how it was unflexible and hard to test an object without using the Dependecy Injection. Also, we have learned more about the evolution of the method to implement the dependencies into the dependent, first by implementing the dependencies into the dependent, then changing the method by passing them manually into the constructor to arrive with the injector system. This then resolves the dependencies, injecting them in the constructor automatically by resolving a tree, which is how Nest.js uses this pattern.
In the next chapter we will see how Nest.js uses TypeORM, an Object Relational Mapping (ORM) that works with several different relational databases.
