What is Dependency Injection?
A design pattern where a component receives its dependencies from the outside rather than creating them internally. This makes code more modular, testable, and flexible.
In plain English
Dependency injection is like hiring a contractor and providing them with tools instead of expecting them to buy their own. If you want to swap out a power drill for a different model, you hand the contractor the new one — you don't need the contractor to know where the tool shop is.
How it works
Instead of a function or class creating its own dependencies (like database connections, API clients, or services), those dependencies are passed in as parameters. For example, instead of a UserService importing and creating its own DatabaseClient, it receives one as a constructor parameter. This lets you easily swap implementations — pass a real database in production and a mock in tests.
Why it matters for AI-built apps
AI-generated code almost always hardcodes dependencies — importing database clients directly, creating HTTP clients inline, and coupling business logic to specific implementations. This makes the code nearly impossible to unit test (you can't test a function without hitting the real database) and painful to change (swapping a database means rewriting every file that imports it).
Common issues
Hardcoded imports of database clients and external services, inability to unit test without real infrastructure, tight coupling between business logic and data access, difficulty swapping implementations (e.g., changing email providers), and circular dependency issues from complex import chains.
Best practices
Pass dependencies as function parameters or constructor arguments. Use interfaces/types to define dependency contracts. In React, use Context for dependency injection. Create factory functions that wire up dependencies at the application boundary. For testing, pass mock implementations that follow the same interface. Consider a lightweight DI container (like tsyringe) for complex applications.
Frequently asked questions
Is dependency injection relevant for frontend React code?
Yes! React's Context API is essentially dependency injection. Custom hooks that accept service parameters are another form. For example, instead of a useAuth hook that imports Supabase directly, create one that receives an auth service parameter — making it testable and swappable. Props themselves are dependency injection at the component level.
Isn't dependency injection over-engineering for a startup?
Not when done simply. You don't need a DI framework — just pass dependencies as parameters instead of hardcoding imports. This takes almost zero extra effort but makes your code testable and flexible. The cost of not doing it becomes apparent when you need to write tests or switch a service provider.
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