What Is Dependency Injection and Why Use It in Go

Dependency Injection (DI) is a design pattern where dependencies are passed into a function or struct rather than created internally, enabling loose coupling and easier testing. In Go, you implement DI by defining interfaces for dependencies and passing concrete implementations via constructor functions or struct fields.

type Service interface {
    Process() error
}

type Worker struct {
    svc Service
}

func NewWorker(svc Service) *Worker {
    return &Worker{svc: svc}
}

This approach lets you swap real implementations for mocks during testing without changing the Worker code.

Dependency Injection is like handing a worker the specific tools they need to do a job instead of forcing them to build those tools themselves. It matters because it lets you swap out parts of your program easily, such as replacing a real database with a fake one for testing. You use it whenever you want your code to be flexible and easy to test.