명령형

const addItem = (todoList, subject, date) => {
  const todoItem = toTodoItem(subject, date)
  todoList.push(todoItem)
}
const printTodoList = (todoList) => {
  const {length} = todoList
  let log = ``
  for (let i = 0; i < length; i++) {
    const todoItem = todoList[i]
    log += `${todoItemToString(todoItem)}`
    if (i < length - 1) {
      log += `\n`
    }
  }
  console.log(log)
}

const toTodoItem = (subject, date) => ({subject, date})
const todoItemToString = ({subject, date}) => `[${date}] ${subject}`

const todoList = []

addItem(todoList, '새해인사', 20200101)
addItem(todoList, '화분 물주기', 20200102)
addItem(todoList, '분리수거', 20200103)

printTodoList(todoList)

/* 실행결과
[20200101] 새해인사
[20200102] 화분 물주기
[20200103] 분리수거
*/

함수형 1

class TodoList {
  constructor(todoList) {
    this.todoList = todoList
  }
  addItem(subject, date) {
    const todoItem = TodoItem.create(subject, date)
    const addedTodoList = this.todoList.concat(todoItem)
    return TodoList.create(addedTodoList)
  }
  toString() {
    return this.todoList
      .map(TodoItem.todoItemToString)
      .join('\n')
  }
  static create(todoList) {
    return new TodoList(todoList)
  }
}

class TodoItem {
  static create (subject, date) {
    return {subject, date}
  }
  static todoItemToString ({subject, date}) {
    return `[${date}] ${subject}`
  }
}

const todoList = TodoList.create([])
  .addItem('새해인사', 20200101)
  .addItem('화분 물주기', 20200102)
const todoList2 = todoList
  .addItem('분리수거', 20200103)

console.group('todoList')
console.log(todoList.toString())
console.groupEnd()

console.group('todoList2')
console.log(todoList2.toString())
console.groupEnd()

함수형 2

const addItem = (todoList, [subject, date]) => {
  return todoList.concat({subject, date})
}
const todoListToString = (todoList) => {
  return todoList
    .map(({subject, date}) => `[${date}] ${subject}`)
    .join('\n')
}

const todoList = [
  ['새해인사', 20200101],
  ['화분 물주기', 20200102],
]
  .reduce(addItem, [])

const todoList2 = addItem(todoList, ['분리수거', 20200103])

console.group('todoList')
console.log(todoListToString(todoList))
console.groupEnd()

console.group('todoList2')
console.log(todoListToString(todoList2))
console.groupEnd()

/* 실행결과
todoList
  [20200101] 새해인사
  [20200102] 화분 물주기
todoList2
  [20200101] 새해인사
  [20200102] 화분 물주기
  [20200103] 분리수거
 */

객체지향

class TodoList {
  constructor() {
    this.todoList = []
  }
  addItem(subject, date) {
    const todoItem = new TodoItem(subject, date)
    this.todoList.push(todoItem)
  }
  print() {
    const str = this.todoList
      .map((todoItem) => todoItem.toString())
      .join('\n')
    console.log(str)
  }
}

class TodoItem {
  constructor(subject, date) {
    this.subject = subject
    this.date = date
  }
  toString() {
    return `[${this.date}] ${this.subject}`
  }
}

const todoList = new TodoList()
todoList.addItem('새해인사', 20200101)
todoList.addItem('화분물주기', 20200102)
todoList.addItem('분리수거', 20200103)
todoList.print()
/*
[20200101] 새해인사
[20200102] 화분물주기
[20200103] 분리수거
*/