IndexedDB学习二：使用
目标
打开数据库连接，开始一个事物创建存储对象请求数据库操作：比如添加或查询数据。等待操作完成， 监听正确的DOM事件。对结果进行其它操作。

创建和构建数据存储使用一个稳定版本的IndexedDB

打开数据库
创建和更新数据库版本

onupgradeneeded是修改数据结构的唯一方法。
createObjectStore是创建数据对象的方法。例子中可以通过SSN获取存储对象。添加和删除数据
使用游标
使用索引
设定游标范围和移动方向
当WEB应用在另一个TAB中打开时更改版本
var openReq = mozIndexedDB.open("MyTestDatabase", 2); openReq.onblocked = function(event) {  // If some other tab is loaded with the database, then it needs to be closed  // before we can proceed.  alert("Please close all other tabs with this site open!");};   openReq.onupgradeneeded = function(event) {  // All other databases have been closed. Set everything up.  db.createObjectStore(/* ... */);  useDatabase(db);}     openReq.onsuccess = function(event) {  var db = event.target.result;  useDatabase(db);  return;} function useDatabase(db) {  // Make sure to add a handler to be notified if another page requests a version  // change. We must close the database. This allows the other page to upgrade the database.  // If you don't do this then the upgrade won't happen until the user close the tab.  db.onversionchange = function(event) {    db.close();    alert("A new version of this page is ready. Please reload!");  };   // Do stuff with the database.

安全

完整的实例
https://developer.mozilla.org/en-US/docs/IndexedDB/Using_IndexedDB