https://dev.to/patroza/comment/mil2

todos.ts

// 3. Presentation/Root
const start = () => {
  const getCompletedTodosRequestHandler = async (ctx) => {
    const result = await getCompletedTodosResolved(ctx.params.id)
    ctx.body = JSON.stringify(result)
  }

  const completeTodoRequestHandler = async (ctx) => {
    await completeTodoResolved(ctx.params.id)
    ctx.status = 204
  }

  const url = "some-url" 
  const getTodoApiResolved = getTodoApi(url)
  const getTodosApiResolved = getTodosApi(url)
  const saveTodoResolved = saveTodoApi(url)
  
  const getCompletedTodosResolved = getCompletedTodos(getTodosApiResolved)
  const completeTodoResolved = completeTodo(getTodoApiResolved, saveTodoResolved)
  app.get("/users/:id/todos", getCompletedTodosRequestHandler)
  app.post("/todos/:id/complete", completeTodoRequestHandler)
  app.listen(3000)
}
//start()

// 3. Infra
let todoStubs = [
  { id: 1, title: "Test", isCompleted: false, updatedAt: new Date(2020, 1, 1)},
  { id: 2, title: "Test 2", isCompleted: true, updatedAt: new Date(2020, 1, 1)},
]

const getTodosApi = (url: string) => async function (userId: number): Promise<Todo[]> {
  return todoStubs
}

const getTodoApi = (url: string) => async function (todoId: number): Promise<Todo | undefined> {
  return todoStubs.find(x => x.id === todoId)
}

const saveTodoApi = (url: string) => async function (todo: Todo): Promise<void> {
  todoStubs = todoStubs.filter(x => x.id === todo.id).concat([todo])
}


// 2. Application
const getCompletedTodos = (getTodos: ReturnType<typeof getTodosApi>) =>
  async function (userId: number): Promise<TodoView> {
    const allTodos = await getTodos(userId)
    const completedTodos = allTodos.filter(x => x.isCompleted)
    return {
      items: completedTodos,
      userId,
    }
  }

const completeTodo = (getTodo: ReturnType<typeof getTodoApi>, saveTodo: ReturnType<typeof saveTodoApi>) =>
  async function (todoId: number): Promise<void> {
    const todo = await getTodo(todoId)
    const updatedTodo = complete(todo, new Date())
    await saveTodo(updatedTodo)
  }

type TodoView = {
  items: Todo[]
  userId: number
}


// 1. Domain
type Todo = {
  readonly id: number
  readonly title: string
  readonly isCompleted: boolean
  readonly updatedAt: Date
}

const complete = (todo: Todo, currentDate: Date) => {
  if (todo.isCompleted) {
    throw new Error("already completed")
  }
  return {
    ...todo,
    isCompleted: true,
    updatedAt: currentDate,
  }
}