Type Aliases for Object Types as Interfaces

queinteresante.js

/* @flow */

///////////////////////////////////////////////////////////
// Types and Classes
//

class User {
  n: number;
  constructor(n) {
    this.n = n;
  }
}

class CustomError {
  message: string;
  constructor(m) {
    this.message = m;
  }
}

type NodeCallback<E, V> = (err: ?E, value: ?V) => void;

type UserDAOIFace = {
  getUserById: (x: number, callback: NodeCallback<CustomError, User>) => void;
}

var userDAOImpl: UserDAOIFace = {
  getUserById(id, callback) {
    /* pretend that we delegate to a MongoDB client or something */
    if (id === 555) {
      callback(new CustomError("User not found!"), undefined);
    }
    else {
      callback(null, new User(id));
    }
  }
}

var fakeUserDAOImpl: UserDAOIFace = {
  getUserById(id, callback) {
    callback(null, new User(555));
  }
}

class UserService {
  dao: UserDAOIFace;
  constructor(dao) {
    this.dao = dao;
  }
  getUserById(userId: number, callback: NodeCallback): void {
    this.dao.getUserById(userId, callback);
  }
}

///////////////////////////////////////////////////////////
// Usage
//

var u = new UserService(userDAOImpl);

u.getUserById(444, function(err, user) {
  console.log(JSON.stringify(user));
});

u.getUserById(555, function(err, user) {
  if (err) {
    console.log(JSON.stringify(err.message));
  }
});