How does DI help with application modularity?

DI makes an application modular because it separates the logic of creating dependencies from using them. Components and services become independent, they can be easily changed, moved, or tested separately.

#Point by point:

1. Each module manages its own dependencies. In Angular you can specify which services are available only within a specific module, which isolates them from the rest of the application.
2. Code reuse. The same service can be used in different modules without duplication, DI figures out on its own where and how to create it.
3. Easy swapping of implementations. If you need to change how a service works (for example, replace a fake with a real API), it is enough to register a different implementation, the component's code stays the same.
4. Painless testing. You can substitute a test dependency for the real one, DI swaps it in automatically when the test runs.

Summary: DI helps make an application flexible and modular because it separates:

• what a class needs,
• and how that is created and managed.