// 存储数据
localStorage.setItem('username', '张三')
localStorage.setItem('settings', JSON.stringify({ theme: 'dark', lang: 'zh' }))

// 读取数据
const username = localStorage.getItem('username') // "张三"
const settings = JSON.parse(localStorage.getItem('settings'))

// 删除数据
localStorage.removeItem('username')

// 清空所有数据
localStorage.clear()

// 获取数据条数
console.log(localStorage.length)

// 获取第 n 个键名
console.log(localStorage.key(0))

// API 与 localStorage 完全相同
sessionStorage.setItem('tempData', 'value')
sessionStorage.getItem('tempData')
sessionStorage.removeItem('tempData')
sessionStorage.clear()

window.addEventListener('storage', (event) => {
  console.log('Key:', event.key)
  console.log('旧值:', event.oldValue)
  console.log('新值:', event.newValue)
  console.log('来源:', event.url)
})

const storage = {
  set(key, value, expires) {
    const data = {
      value,
      expires: expires ? Date.now() + expires : null,
    }
    localStorage.setItem(key, JSON.stringify(data))
  },

  get(key) {
    const item = localStorage.getItem(key)
    if (!item) return null

    const data = JSON.parse(item)

    // 检查过期
    if (data.expires && Date.now() > data.expires) {
      localStorage.removeItem(key)
      return null
    }

    return data.value
  },

  remove(key) {
    localStorage.removeItem(key)
  },

  clear() {
    localStorage.clear()
  },
}

// 使用
storage.set('token', 'abc123', 7 * 24 * 60 * 60 * 1000) // 7天过期
storage.get('token')

┌─────────────────────────────────────────┐
│              IndexedDB                   │
│  ┌────────────────────────────────────┐ │
│  │           Database                  │ │
│  │  ┌──────────────┬───────────────┐  │ │
│  │  │ Object Store │ Object Store  │  │ │
│  │  │   (users)    │   (posts)     │  │ │
│  │  │  ┌────────┐  │  ┌─────────┐  │  │ │
│  │  │  │ Record │  │  │ Record  │  │  │ │
│  │  │  │ Record │  │  │ Record  │  │  │ │
│  │  │  │ Record │  │  │ Record  │  │  │ │
│  │  │  └────────┘  │  └─────────┘  │  │ │
│  │  └──────────────┴───────────────┘  │ │
│  └────────────────────────────────────┘ │
└─────────────────────────────────────────┘

const request = indexedDB.open('MyDatabase', 1)

request.onerror = (event) => {
  console.error('数据库打开失败:', event.target.error)
}

request.onsuccess = (event) => {
  const db = event.target.result
  console.log('数据库打开成功')
}

// 首次创建或版本升级时触发
request.onupgradeneeded = (event) => {
  const db = event.target.result

  // 创建 Object Store
  if (!db.objectStoreNames.contains('users')) {
    const store = db.createObjectStore('users', {
      keyPath: 'id',
      autoIncrement: true,
    })

    // 创建索引
    store.createIndex('email', 'email', { unique: true })
    store.createIndex('name', 'name', { unique: false })
  }
}

// 假设 db 是已打开的数据库实例

// ===== 添加数据 =====
function addUser(user) {
  const transaction = db.transaction(['users'], 'readwrite')
  const store = transaction.objectStore('users')

  const request = store.add(user)

  request.onsuccess = () => {
    console.log('添加成功，ID:', request.result)
  }

  request.onerror = () => {
    console.error('添加失败:', request.error)
  }
}

addUser({ name: '张三', email: 'zhang@example.com', age: 25 })

// ===== 读取数据 =====
function getUser(id) {
  const transaction = db.transaction(['users'], 'readonly')
  const store = transaction.objectStore('users')

  const request = store.get(id)

  request.onsuccess = () => {
    console.log('用户:', request.result)
  }
}

// ===== 更新数据 =====
function updateUser(user) {
  const transaction = db.transaction(['users'], 'readwrite')
  const store = transaction.objectStore('users')

  const request = store.put(user) // put 会更新或插入

  request.onsuccess = () => {
    console.log('更新成功')
  }
}

updateUser({ id: 1, name: '张三', email: 'zhangsan@example.com', age: 26 })

// ===== 删除数据 =====
function deleteUser(id) {
  const transaction = db.transaction(['users'], 'readwrite')
  const store = transaction.objectStore('users')

  const request = store.delete(id)

  request.onsuccess = () => {
    console.log('删除成功')
  }
}

function getAllUsers() {
  const transaction = db.transaction(['users'], 'readonly')
  const store = transaction.objectStore('users')

  const request = store.getAll()

  request.onsuccess = () => {
    console.log('所有用户:', request.result)
  }
}

function iterateUsers() {
  const transaction = db.transaction(['users'], 'readonly')
  const store = transaction.objectStore('users')

  const request = store.openCursor()

  request.onsuccess = (event) => {
    const cursor = event.target.result
    if (cursor) {
      console.log('用户:', cursor.value)
      cursor.continue() // 移动到下一条
    } else {
      console.log('遍历完成')
    }
  }
}

function getUserByEmail(email) {
  const transaction = db.transaction(['users'], 'readonly')
  const store = transaction.objectStore('users')
  const index = store.index('email')

  const request = index.get(email)

  request.onsuccess = () => {
    console.log('查询结果:', request.result)
  }
}

getUserByEmail('zhang@example.com')

class IDBHelper {
  constructor(dbName, version) {
    this.dbName = dbName
    this.version = version
    this.db = null
  }

  async open(upgradeCallback) {
    return new Promise((resolve, reject) => {
      const request = indexedDB.open(this.dbName, this.version)

      request.onerror = () => reject(request.error)
      request.onsuccess = () => {
        this.db = request.result
        resolve(this.db)
      }

      request.onupgradeneeded = (event) => {
        upgradeCallback(event.target.result)
      }
    })
  }

  async add(storeName, data) {
    return new Promise((resolve, reject) => {
      const transaction = this.db.transaction([storeName], 'readwrite')
      const store = transaction.objectStore(storeName)
      const request = store.add(data)

      request.onsuccess = () => resolve(request.result)
      request.onerror = () => reject(request.error)
    })
  }

  async get(storeName, key) {
    return new Promise((resolve, reject) => {
      const transaction = this.db.transaction([storeName], 'readonly')
      const store = transaction.objectStore(storeName)
      const request = store.get(key)

      request.onsuccess = () => resolve(request.result)
      request.onerror = () => reject(request.error)
    })
  }

  async getAll(storeName) {
    return new Promise((resolve, reject) => {
      const transaction = this.db.transaction([storeName], 'readonly')
      const store = transaction.objectStore(storeName)
      const request = store.getAll()

      request.onsuccess = () => resolve(request.result)
      request.onerror = () => reject(request.error)
    })
  }

  async put(storeName, data) {
    return new Promise((resolve, reject) => {
      const transaction = this.db.transaction([storeName], 'readwrite')
      const store = transaction.objectStore(storeName)
      const request = store.put(data)

      request.onsuccess = () => resolve(request.result)
      request.onerror = () => reject(request.error)
    })
  }

  async delete(storeName, key) {
    return new Promise((resolve, reject) => {
      const transaction = this.db.transaction([storeName], 'readwrite')
      const store = transaction.objectStore(storeName)
      const request = store.delete(key)

      request.onsuccess = () => resolve()
      request.onerror = () => reject(request.error)
    })
  }
}

// 使用
const db = new IDBHelper('MyApp', 1)

await db.open((database) => {
  database.createObjectStore('todos', { keyPath: 'id', autoIncrement: true })
})

await db.add('todos', { title: '学习 IndexedDB', completed: false })
const todos = await db.getAll('todos')
console.log(todos)

// 打开缓存
const cache = await caches.open('my-cache-v1')

// 缓存请求
await cache.add('/api/data') // 发起请求并缓存
await cache.addAll(['/style.css', '/app.js', '/logo.png'])

// 手动存入
await cache.put('/api/user', new Response(JSON.stringify({ name: '张三' })))

// 读取缓存
const response = await cache.match('/api/data')
if (response) {
  const data = await response.json()
  console.log(data)
}

// 删除缓存
await cache.delete('/api/data')

// 列出所有缓存的请求
const keys = await cache.keys()
console.log(keys)

// 缓存优先（适合静态资源）
async function cacheFirst(request) {
  const cached = await caches.match(request)
  if (cached) return cached

  const response = await fetch(request)
  const cache = await caches.open('static-v1')
  cache.put(request, response.clone())
  return response
}

// 网络优先（适合 API 数据）
async function networkFirst(request) {
  try {
    const response = await fetch(request)
    const cache = await caches.open('api-v1')
    cache.put(request, response.clone())
    return response
  } catch (error) {
    return caches.match(request)
  }
}

// 仅缓存
async function cacheOnly(request) {
  return caches.match(request)
}

// 仅网络
async function networkOnly(request) {
  return fetch(request)
}

const CACHE_NAME = 'my-cache-v2'

// 删除旧版本缓存
async function cleanOldCaches() {
  const keys = await caches.keys()
  const oldCaches = keys.filter((key) => key !== CACHE_NAME)

  await Promise.all(oldCaches.map((key) => caches.delete(key)))
}

<!doctype html>
<html lang="zh-Hans">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>离线 Todo</title>
    <style>
      * {
        box-sizing: border-box;
        margin: 0;
        padding: 0;
      }
      body {
        font-family: system-ui, sans-serif;
        padding: 2rem;
        max-width: 500px;
        margin: 0 auto;
      }
      h1 {
        margin-bottom: 1.5rem;
      }
      .input-group {
        display: flex;
        gap: 0.5rem;
        margin-bottom: 1.5rem;
      }
      input[type='text'] {
        flex: 1;
        padding: 0.75rem;
        border: 2px solid #e2e8f0;
        border-radius: 0.5rem;
      }
      button {
        padding: 0.75rem 1.5rem;
        background: #3b82f6;
        color: white;
        border: none;
        border-radius: 0.5rem;
        cursor: pointer;
      }
      button:hover {
        background: #2563eb;
      }
      .todo-list {
        list-style: none;
      }
      .todo-item {
        display: flex;
        align-items: center;
        gap: 1rem;
        padding: 1rem;
        border-bottom: 1px solid #e2e8f0;
      }
      .todo-item.completed span {
        text-decoration: line-through;
        color: #94a3b8;
      }
      .todo-item span {
        flex: 1;
      }
      .todo-item input[type='checkbox'] {
        width: 1.25rem;
        height: 1.25rem;
      }
      .delete-btn {
        background: #ef4444;
        padding: 0.5rem 1rem;
      }
      .status {
        margin-top: 1rem;
        padding: 0.75rem;
        border-radius: 0.5rem;
        font-size: 0.875rem;
      }
      .online {
        background: #dcfce7;
        color: #166534;
      }
      .offline {
        background: #fef3c7;
        color: #92400e;
      }
    </style>
  </head>
  <body>
    <h1>📝 离线 Todo</h1>

    <div class="input-group">
      <input type="text" id="todoInput" placeholder="添加新任务..." />
      <button onclick="addTodo()">添加</button>
    </div>

    <ul class="todo-list" id="todoList"></ul>

    <div class="status" id="status"></div>

    <script>
      let db

      // 初始化数据库
      async function initDB() {
        return new Promise((resolve, reject) => {
          const request = indexedDB.open('TodoApp', 1)

          request.onerror = () => reject(request.error)

          request.onsuccess = () => {
            db = request.result
            resolve(db)
          }

          request.onupgradeneeded = (event) => {
            const database = event.target.result
            if (!database.objectStoreNames.contains('todos')) {
              const store = database.createObjectStore('todos', {
                keyPath: 'id',
                autoIncrement: true,
              })
              store.createIndex('createdAt', 'createdAt')
            }
          }
        })
      }

      // 添加 Todo
      async function addTodo() {
        const input = document.getElementById('todoInput')
        const text = input.value.trim()
        if (!text) return

        const todo = {
          text,
          completed: false,
          createdAt: Date.now(),
        }

        await new Promise((resolve, reject) => {
          const transaction = db.transaction(['todos'], 'readwrite')
          const store = transaction.objectStore('todos')
          const request = store.add(todo)
          request.onsuccess = () => resolve(request.result)
          request.onerror = () => reject(request.error)
        })

        input.value = ''
        renderTodos()
      }

      // 切换完成状态
      async function toggleTodo(id) {
        const transaction = db.transaction(['todos'], 'readwrite')
        const store = transaction.objectStore('todos')

        const todo = await new Promise((resolve) => {
          store.get(id).onsuccess = (e) => resolve(e.target.result)
        })

        todo.completed = !todo.completed

        await new Promise((resolve) => {
          store.put(todo).onsuccess = () => resolve()
        })

        renderTodos()
      }

      // 删除 Todo
      async function deleteTodo(id) {
        await new Promise((resolve, reject) => {
          const transaction = db.transaction(['todos'], 'readwrite')
          const store = transaction.objectStore('todos')
          const request = store.delete(id)
          request.onsuccess = () => resolve()
          request.onerror = () => reject(request.error)
        })

        renderTodos()
      }

      // 获取所有 Todo
      async function getAllTodos() {
        return new Promise((resolve, reject) => {
          const transaction = db.transaction(['todos'], 'readonly')
          const store = transaction.objectStore('todos')
          const request = store.getAll()
          request.onsuccess = () => resolve(request.result)
          request.onerror = () => reject(request.error)
        })
      }

      // 渲染列表
      async function renderTodos() {
        const todos = await getAllTodos()
        const list = document.getElementById('todoList')

        // 按创建时间倒序
        todos.sort((a, b) => b.createdAt - a.createdAt)

        list.innerHTML = todos
          .map(
            (todo) => `
        <li class="todo-item ${todo.completed ? 'completed' : ''}">
          <input
            type="checkbox"
            ${todo.completed ? 'checked' : ''}
            onchange="toggleTodo(${todo.id})"
          />
          <span>${escapeHtml(todo.text)}</span>
          <button class="delete-btn" onclick="deleteTodo(${todo.id})">删除</button>
        </li>
      `
          )
          .join('')
      }

      // 转义 HTML
      function escapeHtml(text) {
        const div = document.createElement('div')
        div.textContent = text
        return div.innerHTML
      }

      // 更新网络状态
      function updateNetworkStatus() {
        const status = document.getElementById('status')
        if (navigator.onLine) {
          status.textContent = '🟢 在线 - 数据已保存到本地'
          status.className = 'status online'
        } else {
          status.textContent = '🟡 离线 - 数据仅保存在本地'
          status.className = 'status offline'
        }
      }

      // 监听键盘事件
      document.getElementById('todoInput').addEventListener('keypress', (e) => {
        if (e.key === 'Enter') addTodo()
      })

      // 监听网络状态
      window.addEventListener('online', updateNetworkStatus)
      window.addEventListener('offline', updateNetworkStatus)

      // 初始化
      initDB().then(() => {
        renderTodos()
        updateNetworkStatus()
      })
    </script>
  </body>
</html>

需要存储什么？
│
├─ 少量简单数据（<1MB）
│  ├─ 需要持久化 → localStorage
│  └─ 仅当前会话 → sessionStorage
│
├─ 大量结构化数据
│  └─ IndexedDB
│
├─ 网络请求缓存
│  └─ Cache API
│
└─ 需要服务端读取
   └─ Cookie（但考虑 HttpOnly）

function isStorageAvailable() {
  try {
    const test = '__storage_test__'
    localStorage.setItem(test, test)
    localStorage.removeItem(test)
    return true
  } catch (e) {
    return false
  }
}

if (!isStorageAvailable()) {
  // 降级方案：使用内存存储
  const memoryStorage = new Map()
}

if (navigator.storage && navigator.storage.estimate) {
  const { usage, quota } = await navigator.storage.estimate()
  console.log(`已使用: ${(usage / 1024 / 1024).toFixed(2)} MB`)
  console.log(`总配额: ${(quota / 1024 / 1024).toFixed(2)} MB`)
}

if (navigator.storage && navigator.storage.persist) {
  const persistent = await navigator.storage.persist()
  console.log(persistent ? '持久化存储已授权' : '可能被清除')
}