22: Store Todos Persistent

Let's save the todo list in localStorage and make it persistent.
Define StorageClerk class as module to manage localStorage. It has two static method `fetch` and `save`.
Replace default todos state with result of StorageClerk#fetch.
Define watcher for todos with x-watch directive.
Toggle all collapsed code.
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta
    name="viewport"
    content="width=device-width,initial-scale=1"
  >
  <link
    rel="stylesheet"
    href="https://cdn.jsdelivr.net/npm/todomvc-app-css@2.1.2/index.css"
  >

  <style>
    [data-filter="active"] > .completed {
      display: none;
    }

    [data-filter="completed"] > :not(.completed) {
      display: none;
    }
  </style>

  <script src="https://x-ninja.org/assets/js/o_o.js"></script>
  <script src="https://x-ninja.org/assets/js/flux._.js"></script>
  
  <script>
    class StorageClerk {
      static fetch () {
        const storageJSON = localStorage.getItem(this.STORAGE_KEY)
        return storageJSON
          ? JSON.parse(storageJSON) || []
          : []
      }

      static save (todos) {
        localStorage.setItem(
          this.STORAGE_KEY,
          JSON.stringify(todos)
        )
      }
    }

    StorageClerk.STORAGE_KEY = 'todos-xninjajs'
  </script>
  
  <title>TODOs @x-ninja</title>
</head>
<body>
  <section class="todoapp"
    x-state="{
      todos: [],
      todos: StorageClerk.fetch(),
      editingTodoIndex: -1,
      visibility: 'all',
    }"
    x-invocations="{
      addTodo (_, payload, meta) {
        const newTodo = payload.newTodo.trim()
        if (!newTodo) {
          return
        }

        this.todos.push({
          title: newTodo,
          completed: false
        })
      },
      editTodo (_, payload, meta) {
        const todo = this.todos[payload.index]
        if (!todo) {
          return
        }

        this.beforeEditCache = todo.title
        this.editingTodoIndex = payload.index
      },
      focusTodo (_, payload, meta) {
        const inputs = _.$.querySelectorAll('input.edit')
        const target = inputs[payload.index]
        if (!target) {
          return
        }

        target.focus()
        target.setSelectionRange(
          target.value.length,
          target.value.length
        )
      },
      doneEdit (_, payload, meta) {
        const todo = this.todos[payload.index]
        if (!todo) {
          return
        }

        if (this.editingTodoIndex === -1) {
          return
        }
        this.editingTodoIndex = -1

        const newTitle = meta._.$.value.trim()
        if (newTitle) {
          todo.title = newTitle
        } else {
          _.functions.destroyTodoByIndex(this.todos, payload.index)
        }
      },
      cancelEdit (_, payload, meta) {
        this.editingTodoIndex = -1
      },
      destroyTodo (_, payload, meta) {
        this.todos.splice(payload.index, 1)
      },
      toggleAllTodos (_, payload, meta) {
        this.todos.forEach(
          todo => todo.completed = meta._.$.checked
        )
      },
      clearCompleted (_, payload, meta) {
        this.todos = this.todos.filter(todo => !todo.completed)
      },
      changeVisibility (_, payload, meta) {
        const hash =meta._.$._.attr('href')
        if (!hash) {
          return
        }

        this.visibility = hash.substring(2)
      },
    }"
    x-functions="{
      destroyTodoByIndex (todos, index) {
        todos.splice(index, 1)
      }
    }"
    x-computed="{
      get remaining () {
        return this.todos.filter(todo => !todo.completed).length
      }
    }"
    x-filters="{
      pluralize (count, word) {
        return word + (count === 1 ? '' : 's')
      },
    }"
    x-watch="{
      todos (_, newState, oldState) {
        StorageClerk.save(newState.todos)
      }
    }"
  ::>
    <header class="header">
      <h1>todos</h1>
      <input class="new-todo"
        autofocus
        autocomplete="off"
        placeholder="What needs to be done?"

        @change="addTodo({newTodo: launcher$.value})"
        x-callbacks="{
          change (_) {
            _.$.value = ''
          }
        }"
      >
    </header>
    <section class="main" x-show="todos.length">
      <input id="toggle-all" class="toggle-all" type="checkbox" @change="toggleAllTodos">
      <label for="toggle-all">Mark all as complete</label>
      <ul class="todo-list"
        x-each="todo, index in todos"
        :data-filter="this.visibility"
      >
        <li
          class="todo"
          :class="{
            completed: todo.completed,
            editing: index === editingTodoIndex,
          }"
        >
          <div class="view">
            <input
              class="toggle"
              type="checkbox"
              x-model="todo.completed"
            >
            <label @dblclick="editTodo({index}), focusTodo({index})">{{ todo.title }}</label>
            <button class="destroy" @click="destroyTodo({index})"></button>
          </div>
          <input class="edit"
            :value="todo.title"
            @change="doneEdit({index})"
            @keyup.esc="cancelEdit"
          >
        </li>
      </ul>
    </section>
    <footer class="footer" x-show="todos.length">
      <span class="todo-count">
        <strong x-text="remaining">3</strong> {{ remaining |> pluralize('item') }} left
      </span>
      <ul class="filters"
        @click.prevent="changeVisibility"
      >
        <li><a href="#/all" :class="{selected: visibility == 'all'}">All</a></li>
        <li><a href="#/active" :class="{selected: visibility == 'active'}">Active</a></li>
        <li><a href="#/completed" :class="{selected: visibility == 'completed'}">Completed</a></li>
      </ul>
      <button class="clear-completed"
        @click="clearCompleted"
        x-show="todos.length > remaining"
      >
        Clear completed
      </button>
    </footer>
  </section>
</body>
</html>