todoModel.js

var app = app || {}

;(function () {
  'use strict'

  let Utils = app.Utils
  // Generic "model" object. You can use whatever
  // framework you want. For this application it
  // may not even be worth separating this logic
  // out, but we do this to demonstrate one way to
  // separate out parts of your application.
  app.TodoModel = function (key) {
    this.key = key
    this.todos = Utils.store(key)
    this.onChanges = []
  }

  app.TodoModel.prototype.subscribe = function (onChange) {
    this.onChanges.push(onChange)
  }

  app.TodoModel.prototype.inform = function () {
    Utils.store(this.key, this.todos)
    this.onChanges.forEach(function (cb) {
      cb()
    })
  }

  app.TodoModel.prototype.addTodo = function (...titles) {
    titles.forEach(title => {
      this.todos = this.todos.concat({
        id: Utils.uuid(),
        title,
        completed: false
      })
    })

    this.inform()
  }

  app.TodoModel.prototype.toggleAll = function (checked) {
    // Note: it's usually better to use immutable data structures since they're
    // easier to reason about and React works very well with them. That's why
    // we use map() and filter() everywhere instead of mutating the array or
    // "todo" items themselves.
    this.todos = this.todos.map(function (todo) {
      return Utils.extend({}, todo, { completed: checked })
    })

    this.inform()
  }

  app.TodoModel.prototype.toggle = function (todoToToggle) {
    this.todos = this.todos.map(function (todo) {
      return todo !== todoToToggle
        ? todo
        : Utils.extend({}, todo, { completed: !todo.completed })
    })

    this.inform()
  }

  app.TodoModel.prototype.destroy = function (todo) {
    this.todos = this.todos.filter(function (candidate) {
      return candidate !== todo
    })

    this.inform()
  }

  app.TodoModel.prototype.save = function (todoToSave, text) {
    this.todos = this.todos.map(function (todo) {
      return todo !== todoToSave
        ? todo
        : Utils.extend({}, todo, { title: text })
    })

    this.inform()
  }

  app.TodoModel.prototype.clearCompleted = function () {
    this.todos = this.todos.filter(function (todo) {
      return !todo.completed
    })

    this.inform()
  }
})()