How to Design for Testability with DI in Go

Design for testability in Go by defining interfaces for external dependencies and injecting them into your functions or structs. This allows you to swap real implementations with mocks during testing without changing your production code.

type Repository interface {
    Get(id string) (*User, error)
}

type UserService struct {
    repo Repository
}

func NewUserService(repo Repository) *UserService {
    return &UserService{repo: repo}
}

func (s *UserService) GetUser(id string) (*User, error) {
    return s.repo.Get(id)
}

In tests, you pass a mock implementation of Repository to NewUserService instead of the real database client.

Dependency Injection means giving your code the tools it needs to run, rather than forcing it to build them itself. This is like bringing your own power cord to a device so you can plug it into a wall or a battery depending on the situation. It makes testing easier because you can plug in a fake battery (mock) to check if the device works without needing a real wall outlet.