Snippet for my Medium article

todosService.js

import assert from 'assert'

// Using object destructuring to make it look good.
export function makeTodosService({
  // "repository" is a fancy term to descibe an object
  // that is used to retrieve data from a datasource - the actual
  // data source does not matter. Could be a database, a REST API,
  // or some IoT things like sensors or whatever.
  todosRepository,
  // We also want info about the user that is using the service,
  // so we can restrict access to only their own todos.
  currentUser
}) {
  // Let's assert that we got what we needed.
  assert(todosRepository, 'opts.todosRepository is required.')
  assert(currentUser, 'opts.currentUser is required.')
  return {
    // Gets todos for the current users.
    getTodos: async (query) => {
      // Query is just an object. In this case we can declare what
      // todos we want - completed, uncompleted, or all.
      const todos = await todosRepository.find({
        // can be ALL, INCOMPLETED, COMPLETED
        filter: query.filter,
        userId: currentUser.id
      })
      return todos
    },
    
    // Creates a new todo for the current user
    createTodo: async (data) => {
      const newTodo = await todosRepository.create({
        text: data.text,
        userId: currentUser.id,
        completed: false
      })
      
      return newTodo
    },
    
    // Updates a todo if allowed
    updateTodo: async (todoId, data) {
      const todo = await todosRepository.get(todoId)
      
      // Verify that we are allowed to modify this todo
      if (todo.userId !== currentUser.id) {
        throw new Error('Forbidden!')
      }
      
      const updatedTodo = await todosRepository.update(todoId, {
        text: data.text,
        completed: data.completed
      })
      
      return updatedTodo
    },
    
    // Deletes a todo if allowed
    deleteTodo: async (todoId) {
      const todo = await todosRepository.get(todoId)
      
      // Verify that we are allowed to delete this todo
      if (todo.userId !== currentUser.id) {
        throw new Error('Forbidden!')
      }
      
      await todosRepository.delete(todoId)
    }
  }
}