TodoListModel.js

import { observable, computed, action } from 'mobx'

import TodoModel from './TodoModel'

export default class TodoListModel {
  @observable todos = []
  
  constructor () {
    try {
      this.todos = JSON.parse(window.localStorage.getItem('todos'))
    } catch (e) {
      /* swallow */
    }
  }

  @computed
  get finishedTodoCount () {
    return this.todos.filter(todo => todo.finished).length
  }
  
  @computed
  get percentComplete () {
    const finished = this.todos.filter(todo => todo.finished).length
    return Math.floor(finished/this.todos.length * 100) || 0
  }

  @action
  addTodo (title) {
    this.todos.push(new TodoModel(title))
    window.localStorage.setItem('todos', JSON.stringify(this.todos))
  }
}

TodoModel.js

import { observable } from 'mobx'

export default class TodoModel {
  @observable title
  @observable finished = false

  constructor (title) {
    this.title = title
  }
}