Can you explain how dependency injection works in AngularJS?

Can you explain how dependency injection works in AngularJS?

In AngularJS, dependency injection is a design pattern that allows the developer to inject dependencies, such as services, into a component (such as a controller or directive) instead of hard-coding them within the component. This makes the component more flexible and easier to test, as the dependencies can be easily swapped out or mocked.

Dependency injection in AngularJS is implemented using a special injector service, which is responsible for instantiating the dependencies and providing them to the components that need them. The injector uses a set of provider objects to determine how to create the dependencies. Each provider object contains information about a specific dependency, such as its name, type, and the factory function that creates it.

When a component requests a dependency, the injector looks up the corresponding provider object and uses the factory function to create an instance of the dependency. The instance is then cached, so that subsequent requests for the same dependency return the same instance.

The dependency injection system in AngularJS also supports the concept of "dependency injection tokens", which are used to identify dependencies. This allows dependencies to be specified by name, rather than by type or instance.

To use dependency injection in AngularJS, you need to register the dependencies with the injector service, and then use the $inject property or the inline array annotation to specify the dependencies in the component's constructor function.

Overall, dependency injection in AngularJS helps to increase the reusability, maintainability and testability of code.