NestJS and Dependency Injection

The Dependency Inversion principle tells us that the most flexible system are those in which source code dependencies refer only to abstractions, not concrete implementation. Well, this definition is straightforward for statically typed languages like Java and may be vague or even confusion for dynamically typed languages like JavaScript. We will explore those differences a little bit later comparing how DI implemented in both Spring framework (Java) and NestJS (NodeJS).

Let’s have a look where we would need to apply this pattern and how it may be useful. Any well-designed BackEnd application has strictly defined layers (see picture 1):

The first requirement for the project is that we need to implement User domain and for persistent layer should be used MySQL DB. First of all, User class entity should be defined with all necessary properties (fields). Let’s also declare TS interface for UserRepository and the first its implementation which is supposed to work with MySQL — UserMySQLRepository. UserService is the intermediate layer between controller and repository, which needs to have dependency from UserRepository:

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