Functional Dependency Injection in TypeScript

DI.py

# dataclass saves the trouble of writing __init__ implementations
# frozen=True makes the class instances immutable (so no hidden mutable state)
@dataclass(frozen=True)
class UserConnector:
  def get_user(id: str) -> User:
    return User.objects.get(id=id)
  
@dataclass(frozen=True)
class OrgConnector:
  def get_org(id: str) -> Org:
    return Org.objects.get(id=id)
  
@dataclass(frozen=True)
class UserService:
  # we can pass default implementations here because they are immutable
  # (would be dangerous if not as these would become shared for all instances of UserService)
  user_connector: UserConnector = UserConnector()
  org_connector: OrgConnector = OrgConnector()
  
  def get_user_with_org(self, id: str) -> User:
    return {
        'user': self.user_connector.get_user(id),
        'org': self.org_connector.get_org(id)
    }

if __name__ == '__main__':
    # can be parametrised with Mock user_connector and org_connector in tests
    s = UserService()
    user = s.get_user_with_org("123")

DI.ts

// Connector types (someone might call these DAO)
type UserConnector = {
  getUser: (id: string) => User
}
type OrgConnector = {
  getOrg: (id: string) => Org
}
// Dependencies for User Service
type UserServiceDeps = {
  userConnector: UserConnector
  orgConnector: OrgConnector
}
// "Factory" for user service, just use a closure+object literal (really more like a Record type here)
const makeUserService = ({ userConnector, orgConnector }: UserServiceDeps) => ({
  getUserWithOrg() {
    return {
      user: userConnector.getUser(),
      org: orgConnector.getOrg()
    }
  },
  getUser() {
    return userConnector.getUser()
  }
})
function main() {
  // these are probably in a different file, fancier, can also be parametrised
  const userConnector = { getUser: (id) => { db.select('user').where({id}) }}
  const orgConnector = { getOrg: (id) => { db.select('organisation').where({id}) }}
  // build UserService, inject connectors
  const UserService = makeUserService({ userConnector, orgConnector })
  const user = UserService.getUserWithOrg()
}