使用 IndexedDB

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  • 版本标题: Using IndexedDB
  • 版本 id: 304753
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  • 创建者: karsa.si
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IndexedDB是使用浏览器将大量数据存储到本地的方式,相当于一个key-value数据库. 它可以使得web应用程序大大的提升, 使得你的web程序可以工作在离线. 

IndexedDB 包含一组synchronous(同步) 和一组 asynchronous(异步) API. 同步API现在没有被任何浏览器支持.  这篇文章是说如何使用异步api的.

基本套路(操作流程)

在进行操作以前,请务必将IndexedDB的真实存储文件删除,以保证是一个空的开始环境,不然无法触发onupgradeneeded事件。windows下的存储文件在:C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\<*>.default\indexedDB。

1,打开一个数据库.

假定我们创造的数据库名为db。

window.indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.msIndexedDB;
//多浏览器兼容,标准确定后只需要第一项
window.db=null;// 这个全局变量会用到的,存储数据库对象
window.addEventListener("DOMContentLoaded", contentLoaded, false);
//dom加载完毕后再运行代码可以避免很多不必要的问题
function contentLoaded() {
 var dbOpenRequest= indexedDB.open("db");
 dbOpenRequest.onerror = function (evt) {    // evt.target == dbOpenRequest
  alert("IndexedDB dbOpenRequest error: " + evt.target.error.name);
 }
 dbOpenRequest.onsuccess = function (evt) {
  db = evt.target.result;//db对象就得到了
 }
 dbOpenRequest.onupgradeneeded = function(evt) {    // only creat or version change triger it
  //只有创建或者改变版本的情况下才能触发它,经测试,firefox只有手动删除数据库文件才能促使他
  // 新建立数据库,什么清空缓存之类无效!实验的时候特别注意要删除数据库文件。
  alert ("here")
  // 这里应该填充objectStore建立代码见下。
 }

2, 创造一个object stores存储。

// 假定要存的是人的名字和邮件如下表
var peopleData = [
{ name: "John Dow", email: "john@company.com" },
{ name: "Don Dow", email: "don@company.com" }
];

dbOpenRequest.onupgradeneeded = function(evt) { //接上面代码
  var db = evt.target.result;
  var objectStore = db.createObjectStore("people", { keyPath: "name" });
  objectStore.createIndex("email", "email", { unique: true });
  // Store values in the newly created objectStore.
  for (var i in peopleData) {
    objectStore.add(peopleData [i]);
  }
};

添加数据

var transaction = db.transaction(["customers"], IDBTransaction.readwrite);

transaction()返回一个transaction 对象. 第一个参数是挂钩的store,空表示所有. 如果第二个参数是空,你会得到一个只读的transaction. 因为这里要读写,所以传递了一个 readwrite 标志.

 

 

等待请求结束,通过监听DOM事件的方法(by listening to the right kind of DOM event.)(js编程里很常见的)

使用操作结果。(which can be found on the request object).
 
 
添加和删除数据

在对一个新的数据库做操作以前,必须开始一个事务(transaction). Transactions 和object stores挂钩. transactions 有三种模式(read-only, read/write, and versionchange)。

transaction 的生存周期. Transactions和event loop紧密相连. If you make a transaction and return to the event loop without using it then the transaction will become inactive. The only way to keep the transaction active is to make a request on it. When the request is finished you'll get a DOM event and, assuming that the request succeeded, you'll have another opportunity to extend the transaction during that callback. If you return to the event loop without extending the transaction then it will become inactive, and so on. As long as there are pending requests the transaction remains active. Transaction lifetimes are really very simple but it might take a little time to get used to. A few more examples will help, too. If you start seeing TRANSACTION_INACTIVE_ERR error codes then you've messed something up.

Transactions can receive DOM events of three different types: error, abort, and complete. We've talked about the way that error events bubble, so a transaction  receives error events from any requests that are generated from it. A more subtle point here is that the default behavior of an error is to abort the transaction in which it occurred. Unless you handle the error by calling preventDefault() on the error event, the entire transaction is rolled back. This design forces you to  think about and handle errors, but you can always add a catchall error handler to the database if fine grained error handling is too cumbersome. If you don't handle an error event or if you call abort() on the transaction, then the transaction is rolled back and an abort event is fired on the transaction. Otherwise, after all pending requests have completed, you'll get a complete event. If you're doing lots of database operations, then tracking the transaction rather than individual requests can certainly aide your sanity.

Now that you have a transaction, you'll need to get the object store from it. Transactions only let you have an object store that you specified when creating the transaction. Then you can add all the data you need.

// Do something when all the data is added to the database.
transaction.oncomplete = function(event) {
  alert("All done!");
};

transaction.onerror = function(event) {
  // Don't forget to handle errors!
};

var objectStore = transaction.objectStore("customers");
for (var i in customerData) {
  var request = objectStore.add(customerData[i]);
  request.onsuccess = function(event) {
    // event.target.result == customerData[i].ssn
  };
}

The result of a request generated from a call to add() is the key of the value that was added. So in this case, it should equal the ssn property of the object that was added, since the object store uses the ssn property for the key path. Note that the add() function requires that no object already be in the database with the same key. If you're trying to modify an existing entry, or you don't care if one exists already, use the put() function.

Removing data from the database

Removing data is very similar:

var request = db.transaction(["customers"], IDBTransaction.readwrite)
                .objectStore("customers")
                .delete("444-44-4444");
request.onsuccess = function(event) {
  // It's gone!
};

Getting data from the database

Now that the database has some info in it, you can retrieve it in several ways. First, the simple get(). You need to provide the key to retrieve the value, like so:

var transaction = db.transaction(["customers"]);
var objectStore = transaction.objectStore("customers");
var request = objectStore.get("444-44-4444");
request.onerror = function(event) {
  // Handle errors!
};
request.onsuccess = function(event) {
  // Do something with the request.result!
  alert("Name for SSN 444-44-4444 is " + request.result.name);
};

That's a lot of code for a "simple" retrieval. Here's how you can shorten it up a bit, assuming that you handle errors at the database level:

db.transaction("customers").objectStore("customers").get("444-44-4444").onsuccess = function(event) {
  alert("Name for SSN 444-44-4444 is " + event.target.result.name);
};

See how this works? Since there's only one object store, you can avoid passing a list of object stores you need in your transaction and just pass the name as a string. Also, you're only reading from the database, so you don't need a readwrite transaction. Calling transaction() with no mode specified gives you a readonly transaction. Another subtlety here is that you don't actually save the request object to a variable. Since the DOM event has the request as its target you can use the event to get to the result property. Easy, right?!

Using a cursor

Using get() requires that you know which key you want to retrieve. If you want to step through all the values in your object store, then you can use a cursor. Here's what it looks like:

var objectStore = db.transaction("customers").objectStore("customers");

objectStore.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    alert("Name for SSN " + cursor.key + " is " + cursor.value.name);
    cursor.continue();
  }
  else {
    alert("No more entries!");
  }
};

The openCursor() function takes several arguments. First, you can limit the range of items that are retrieved by using a key range object that we'll get to in a minute. Second, you can specify the direction that you want to iterate. In the above example, we're iterating over all objects in ascending order. The success callback for cursors is a little special. The cursor object itself is the result of the request (above we're using the shorthand, so it's event.target.result). Then the actual key and value can be found on the key and value properties of the cursor object. If you want to keep going, then you have to call continue() on the cursor. When you've reached the end of the data (or if there were no entries that matched your openCursor() request) you still get a success callback, but the result property is undefined.

One common pattern with cursors is to retrieve all objects in an object store and add them to an array, like this:

var customers = [];

objectStore.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    customers.push(cursor.value);
    cursor.continue();
  }
  else {
    alert("Got all customers: " + customers);
  }
};
Warning: The following function is not part of the IndexedDB standard!

Mozilla has also implemented getAll() to handle this case. It isn't part of the IndexedDB standard, so it may disappear in the future. We've included it because we think it's useful. The following code does precisely the same thing as above:

objectStore.getAll().onsuccess = function(event) {
  alert("Got all customers: " + customers);
};

There is a performance cost associated with looking at the value property of a cursor, because the object is created lazily. When you use getAll(), Gecko must create all the objects at once. If you're just interested in looking at each of the keys, for instance, it is much more efficient to use a cursor than to use getAll(). If you're trying to get an array of all the objects in an object store, though, use getAll().

Using an index

Storing customer data using the SSN as a key is logical since the SSN uniquely identifies an individual. (Whether this is a good idea for privacy is a different question, outside the scope of this article.) If you need to look up a customer by name, however, you'll need to iterate over every SSN in the database until you find the right one. Searching in this fashion would be very slow, so instead you can use an index.

var index = objectStore.index("name");
index.get("Donna").onsuccess = function(event) {
  alert("Donna's SSN is " + event.target.result.ssn);
};

The "name" cursor isn't unique, so there could be more than one entry with the name set to "Donna". In that case you always get the one with the lowest key value.

If you need to access all the entries with a given name you can use a cursor. You can open two different types of cursors on indexes. A normal cursor maps the index property to the object in the object store. A key cursor maps the index property to the key used to store the object in the object store. The differences are illustrated here:

index.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // cursor.key is a name, like "Bill", and cursor.value is the whole object.
    alert("Name: " + cursor.key + ", SSN: " + cursor.value.ssn + ", email: " + cursor.value.email);
    cursor.continue();
  }
};

index.openKeyCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // cursor.key is a name, like "Bill", and cursor.value is the SSN.
    // No way to directly get the rest of the stored object.
    alert("Name: " + cursor.key + ", "SSN: " + cursor.value);
    cursor.continue();
  }
};

Specifying the range and direction of cursors

If you would like to limit the range of values you see in a cursor, you can use a key range object and pass it as the first argument to openCursor() or openKeyCursor(). You can make a key range that only allows a single key, or one the has a lower or upper bound, or one that has both a lower and upper bound. The bound may be "closed" (i.e., the key range includes the given value) or "open" (i.e., the key range does not include the given value). Here's how it works:

// Only match "Donna"
var singleKeyRange = IDBKeyRange.only("Donna");

// Match anything past "Bill", including "Bill"
var lowerBoundKeyRange = IDBKeyRange.lowerBound("Bill");

// Match anything past "Bill", but don't include "Bill"
var lowerBoundOpenKeyRange = IDBKeyRange.lowerBound("Bill", true);

// Match anything up to, but not including, "Donna"
var upperBoundOpenKeyRange = IDBKeyRange.upperBound("Donna", true);

//Match anything between "Bill" and "Donna", but not including "Donna"
var boundKeyRange = IDBKeyRange.bound("Bill", "Donna", false, true);

index.openCursor(boundKeyRange).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the matches.
    cursor.continue();
  }
};

Sometimes you may want to iterate in descending order rather than in ascending order (the default direction for all cursors). Switching direction is accomplished by passing prev to the openCursor() function:

objectStore.openCursor(null, IDBCursor.prev).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the entries.
    cursor.continue();
  }
};

Since the "name" index isn't unique, there might be multiple entries where name is the same. Note that such a situation cannot occur with object stores since the key must always be unique. If you wish to filter out duplicates during cursor iteration over indexes, you can pass nextunique (or prevunique if you're going backwards) as the direction parameter. When nextunique or prevunique is used, the entry with the lowest key is always the one returned.

index.openKeyCursor(null, IDBCursor.nextunique).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the entries.
    cursor.continue();
  }
};

Version changes while a web app is open in another tab

When your web app changes in such a way that a version change is required for your database, you need to consider what happens if the user has the old version of your app open in one tab and then loads the new version of your app in another. When you call open() with a greater version than the actual version of the database, all other open databases must explicitly acknowledge the request before you can start making changes to the database. Here's how it works:

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.
}

Security

IndexedDB uses the same-origin principle, which means that it ties the store to the origin of the site that creates it (typically, this is the site domain or subdomain), so it cannot be accessed by any other origin.

It's important to note that IndexedDB doesn't work for content loaded into a frame from another site (either {{ HTMLElement("frame") }} or {{ HTMLElement("iframe") }}. This is a security measure. Details as to why this matters are forthcoming. See {{ bug(595307) }}.

Next step

If you want to start tinkering with the API, jump in to the reference documentation and checking out the different methods.

See also

Reference

Tutorials

Related articles

Firefox

修订版来源

<p>IndexedDB是使用浏览器将大量数据存储到本地的方式,相当于一个key-value数据库. 它可以使得web应用程序大大的提升, 使得你的web程序可以工作在离线.&nbsp;</p>
<p>IndexedDB 包含一组<a href="/en/IndexedDB#Synchronous_API" title="https://developer.mozilla.org/en/IndexedDB#Synchronous_API">synchronous(同步)</a> 和一组 <a href="/en/IndexedDB#Asynchronous_API" title="https://developer.mozilla.org/en/IndexedDB#Asynchronous_API">asynchronous(异步)</a> API. 同步API现在没有被任何浏览器支持.&nbsp; 这篇文章是说如何使用异步api的.</p>
<h2 id="pattern" name="pattern">基本套路(操作流程)</h2>
<p>在进行操作以前,请务必将IndexedDB的真实存储文件删除,以保证是一个空的开始环境,不然无法触发onupgradeneeded事件。windows下的存储文件在:C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\&lt;*&gt;.default\indexedDB。</p>
<p>1,打开一个数据库.</p>
<p>假定我们创造的数据库名为db。</p>
<pre class="brush: js" style="margin-left: 80px;">
window.indexedDB = window.indexedDB || window.webkitIndexedDB || window.mozIndexedDB || window.msIndexedDB;
//多浏览器兼容,标准确定后只需要第一项
window.db=null;// 这个全局变量会用到的,存储数据库对象
window.addEventListener("DOMContentLoaded", contentLoaded, false);
//dom加载完毕后再运行代码可以避免很多不必要的问题
function contentLoaded() {
 var dbOpenRequest= indexedDB.open("db");
 dbOpenRequest.onerror = function (evt) {&nbsp;&nbsp; &nbsp;// evt.target == dbOpenRequest
  alert("IndexedDB dbOpenRequest error: " + evt.target.error.name);
 }
 dbOpenRequest.onsuccess = function (evt) {
  db = evt.target.result;//db对象就得到了
 }
 dbOpenRequest.onupgradeneeded = function(evt) {&nbsp;&nbsp; &nbsp;// only creat or version change triger it
  //只有创建或者改变版本的情况下才能触发它,经测试,firefox只有手动删除数据库文件才能促使他
  // 新建立数据库,什么清空缓存之类无效!实验的时候特别注意要删除数据库文件。
  alert ("here")
  // 这里应该填充objectStore建立代码见下。
 }

</pre>
<p>2, 创造一个object stores存储。</p>
<pre class="brush: js">
// 假定要存的是人的名字和邮件如下表
var peopleData = [
{ name: "John Dow", email: "john@company.com" },
{ name: "Don Dow", email: "don@company.com" }
];

dbOpenRequest.onupgradeneeded = function(evt) { //接上面代码
  var db = evt.target.result;
  var objectStore = db.createObjectStore("people", { keyPath: "name" });
  objectStore.createIndex("email", "email", { unique: true });
  // Store values in the newly created objectStore.
  for (var i in peopleData) {
    objectStore.add(peopleData [i]);
  }
};</pre>
<p>添加数据</p>
<pre class="brush: js">
var transaction = db.transaction(["customers"], IDBTransaction.readwrite);</pre>
<p><code>transaction()</code>返回一个transaction 对象. 第一个参数是挂钩的store,空表示所有. 如果第二个参数是空,你会得到一个只读的transaction. 因为这里要读写,所以传递了一个 <code>readwrite</code> 标志.</p>
<p>&nbsp;</p>
<p>&nbsp;</p>
<p>等待请求结束,通过监听DOM事件的方法(by listening to the right kind of DOM&nbsp;event.)(js编程里很常见的)</p>
<div>
  使用操作结果。(which can be found on the request object).</div>
<div>
  &nbsp;</div>
<div>
  &nbsp;</div>
<div>
  添加和删除数据</div>
<p>在对一个新的数据库做操作以前,必须开始一个事务(transaction). Transactions 和object stores挂钩. transactions 有三种模式(read-only, read/write, and versionchange)。</p>
<p>transaction 的生存周期. Transactions和event loop紧密相连. If you make a transaction and return to the event loop without using it then the transaction will become inactive. The only way to keep the transaction active is to make a request on it. When the request is finished you'll get a DOM&nbsp;event and, assuming that the request succeeded, you'll have another opportunity to extend the transaction during that callback. If you return to the event loop without extending the transaction then it will become inactive, and so on. As long as there are pending requests the transaction remains active. Transaction lifetimes are really very simple but it might take a little time to get used to. A few more examples will help, too. If you start seeing <code>TRANSACTION_INACTIVE_ERR</code> error codes then you've messed something up.</p>
<p>Transactions can receive DOM&nbsp;events of three different types: <code>error</code>, <code>abort</code>, and <code>complete</code>. We've talked about the way that <code>error</code> events bubble, so a transaction&nbsp; receives error events from any requests that are generated from it. A more subtle point here is that the default behavior of an error is to abort the transaction in which it occurred. Unless you handle the error by calling <code>preventDefault()</code> on the error event, the entire transaction is rolled back. This design forces you to&nbsp; think about and handle errors, but you can always add a catchall error handler to the database if fine grained error handling is too cumbersome. If you don't handle an error event or if you call <code>abort()</code> on the transaction, then the transaction is rolled back and an <code>abort</code> event is fired on the transaction. Otherwise, after all pending requests have completed, you'll get a <code>complete</code> event. If you're doing lots of database operations, then tracking the transaction rather than individual requests can certainly aide your sanity.</p>
<p>Now that you have a transaction, you'll need to get the object store from it. Transactions only let you have an object store that you specified when creating the transaction. Then you can add all the data you need.</p>
<pre class="brush: js">
// Do something when all the data is added to the database.
transaction.oncomplete = function(event) {
  alert("All done!");
};

transaction.onerror = function(event) {
  // Don't forget to handle errors!
};

var objectStore = transaction.objectStore("customers");
for (var i in customerData) {
  var request = objectStore.add(customerData[i]);
  request.onsuccess = function(event) {
    // event.target.result == customerData[i].ssn
  };
}</pre>
<p>The <code>result</code> of a request generated from a call to <code>add() </code>is the key of the value that was added. So in this case, it should equal the <code>ssn</code> property of the object that was added, since the object store uses the <code>ssn</code> property for the key path. Note that the <code>add()</code> function requires that no object already be in the database with the same key. If you're trying to modify an existing entry, or you don't care if one exists already, use the <code>put()</code> function.</p>
<h2 id="Removing_data_from_the_database">Removing data from the database</h2>
<p>Removing data is very similar:</p>
<pre class="brush: js">
var request = db.transaction(["customers"], IDBTransaction.readwrite)
                .objectStore("customers")
                .delete("444-44-4444");
request.onsuccess = function(event) {
  // It's gone!
};</pre>
<h2 id="Getting_data_from_the_database">Getting data from the database</h2>
<p>Now that the database has some info in it, you can retrieve it in several ways. First, the simple <code>get()</code>.&nbsp;You need to provide the key to retrieve the value, like so:</p>
<pre class="brush: js">
var transaction = db.transaction(["customers"]);
var objectStore = transaction.objectStore("customers");
var request = objectStore.get("444-44-4444");
request.onerror = function(event) {
  // Handle errors!
};
request.onsuccess = function(event) {
  // Do something with the request.result!
  alert("Name for SSN 444-44-4444 is " + request.result.name);
};</pre>
<p>That's a lot of code for a "simple" retrieval. Here's how you can shorten it up a bit, assuming that you handle errors at the database level:</p>
<pre class="brush: js">
db.transaction("customers").objectStore("customers").get("444-44-4444").onsuccess = function(event) {
  alert("Name for SSN 444-44-4444 is " + event.target.result.name);
};</pre>
<p>See how this works? Since there's only one object store, you can avoid passing a list of object stores you need in your transaction and just pass the name as a string. Also, you're only reading from the database, so you don't need a <code>readwrite</code> transaction. Calling <code>transaction()</code> with no mode specified gives you a <code>readonly</code> transaction. Another subtlety here is that you don't actually save the request object to a variable. Since the DOM event has the request as its target you can use the event to get to the <code>result</code> property.&nbsp;Easy, right?!</p>
<h2 id="Using_a_cursor">Using a cursor</h2>
<p>Using <code>get()</code> requires that you know which key you want to retrieve. If you want to step through all the values in your object store, then you can use a cursor. Here's what it looks like:</p>
<pre class="brush: js">
var objectStore = db.transaction("customers").objectStore("customers");

objectStore.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    alert("Name for SSN " + cursor.key + " is " + cursor.value.name);
    cursor.continue();
  }
  else {
    alert("No more entries!");
  }
};</pre>
<p>The<code> openCursor()</code> function takes several arguments. First, you can limit the range of items that are retrieved by using a key range object that we'll get to in a minute. Second, you can specify the direction that you want to iterate. In the above example, we're iterating over all objects in ascending order. The success callback for cursors is a little special. The cursor object itself is the <code>result</code> of the request (above we're using the shorthand, so it's <code>event.target.result</code>). Then the actual key and value can be found on the <code>key</code> and <code>value</code> properties of the cursor object. If you want to keep going, then you have to call <code>continue()</code> on the cursor. When you've reached the end of the data (or if there were no entries that matched your <code>openCursor()</code> request) you still get a success callback, but the <code>result</code> property is <code>undefined</code>.</p>
<p>One common pattern with cursors is to retrieve all objects in an object store and add them to an array, like this:</p>
<pre class="brush: js">
var customers = [];

objectStore.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    customers.push(cursor.value);
    cursor.continue();
  }
  else {
    alert("Got all customers: " + customers);
  }
};</pre>
<div class="warning">
  <strong>Warning:</strong>&nbsp;The following function is not part of the IndexedDB standard!</div>
<p>Mozilla has also implemented <code>getAll()</code> to handle this case. It isn't part of the IndexedDB standard, so it may disappear in the future. We've included it because we think it's useful. The following code does precisely the same thing as above:</p>
<pre class="brush: js">
objectStore.getAll().onsuccess = function(event) {
  alert("Got all customers: " + customers);
};</pre>
<p>There is a performance cost associated with looking at the <code>value</code> property of a cursor, because the object is created lazily. When you use <code>getAll()</code>, Gecko must create all the objects at once. If you're just interested in looking at each of the keys, for instance, it is much more efficient to use a cursor than to use <code>getAll()</code>. If you're trying to get an array of all the objects in an object store, though, use <code>getAll()</code>.</p>
<h3 id="Using_an_index">Using an index</h3>
<p>Storing customer data using the SSN&nbsp;as a key is logical since the SSN&nbsp;uniquely identifies an individual. (Whether this is a good idea for privacy is a different question, outside the scope of this article.) If you need to look up a customer by name, however, you'll need to iterate over every SSN&nbsp;in the database until you find the right one. Searching in this fashion would be very slow, so instead you can use an index.</p>
<pre class="brush: js">
var index = objectStore.index("name");
index.get("Donna").onsuccess = function(event) {
  alert("Donna's SSN is " + event.target.result.ssn);
};</pre>
<p>The "name" cursor isn't unique, so there could be more than one entry with the <code>name</code> set to <code>"Donna"</code>. In that case you always get the one with the lowest key value.</p>
<p>If you need to access all the entries with a given <code>name</code> you can use a cursor. You can open two different types of cursors on indexes. A normal cursor maps the index property to the object in the object store. A key cursor maps the index property to the key used to store the object in the object store. The differences are illustrated here:</p>
<pre class="brush: js">
index.openCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // cursor.key is a name, like "Bill", and cursor.value is the whole object.
    alert("Name: " + cursor.key + ", SSN: " + cursor.value.ssn + ", email: " +&nbsp;cursor.value.email);
    cursor.continue();
  }
};

index.openKeyCursor().onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // cursor.key is a name, like "Bill", and cursor.value is the SSN.
    // No way to directly get the rest of the stored object.
    alert("Name: " + cursor.key + ", "SSN: " + cursor.value);
    cursor.continue();
  }
};</pre>
<h3 id="Specifying_the_range_and_direction_of_cursors">Specifying the range and direction of cursors</h3>
<p>If you would like to limit the range of values you see in a cursor, you can use a key range object and pass it as the first argument to <code>openCursor()</code> or <code>openKeyCursor()</code>. You can make a key range that only allows a single key, or one the has a lower or upper bound, or one that has both a lower and upper bound. The bound may be "closed" (i.e., the key range includes the given value) or "open" (i.e., the key range does not include the given value). Here's how it works:</p>
<pre class="brush: js">
// Only match "Donna"
var singleKeyRange = IDBKeyRange.only("Donna");

// Match anything past "Bill", including "Bill"
var lowerBoundKeyRange = IDBKeyRange.lowerBound("Bill");

// Match anything past "Bill", but don't include "Bill"
var lowerBoundOpenKeyRange = IDBKeyRange.lowerBound("Bill", true);

// Match anything up to, but not including, "Donna"
var upperBoundOpenKeyRange = IDBKeyRange.upperBound("Donna", true);

//Match anything between "Bill" and "Donna", but not including "Donna"
var boundKeyRange = IDBKeyRange.bound("Bill", "Donna", false, true);

index.openCursor(boundKeyRange).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the matches.
    cursor.continue();
  }
};</pre>
<p>Sometimes you may want to iterate in descending order rather than in ascending order (the default direction for all cursors). Switching direction is accomplished by passing <code>prev</code> to the <code>openCursor()</code> function:</p>
<pre class="brush: js">
objectStore.openCursor(null, IDBCursor.prev).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the entries.
    cursor.continue();
  }
};</pre>
<p>Since the "name" index isn't unique, there might be multiple entries where <code>name</code> is the same. Note that such a situation cannot occur with object stores since the key must always be unique. If you wish to filter out duplicates during cursor iteration over indexes, you can pass <code>nextunique</code>&nbsp;(or <code>prevunique</code>&nbsp;if you're going backwards) as the direction parameter. When <code>nextunique</code> or <code>prevunique</code>&nbsp;is used, the entry with the lowest key is always the one returned.</p>
<pre class="brush: js">
index.openKeyCursor(null, IDBCursor.nextunique).onsuccess = function(event) {
  var cursor = event.target.result;
  if (cursor) {
    // Do something with the entries.
    cursor.continue();
  }
};</pre>
<h2 id="Version_changes_while_a_web_app_is_open_in_another_tab">Version changes while a web app is open in another tab</h2>
<p>When your web app changes in such a way that a version change is required for your database, you need to consider what happens if the user has the old version of your app open in one tab and then loads the new version of your app in another. When you call <code>open()</code> with a greater version than the actual version of the database, all other open databases must explicitly acknowledge the request before you can start making changes to the database. Here's how it works:</p>
<pre class="brush: js">
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.
 &nbsp;db.onversionchange = function(event) {
    db.close();
    alert("A new version of this page is ready. Please reload!");
  };

  // Do stuff with the database.
}
</pre>
<h2 id="Security">Security</h2>
<p>IndexedDB uses the same-origin principle, which means that it ties the store to the origin of the site that creates it (typically, this is the site domain or subdomain), so it cannot be accessed by any other origin.</p>
<p>It's important to note that IndexedDB&nbsp;doesn't work for content loaded into a frame from another site (either {{ HTMLElement("frame") }}&nbsp;or {{ HTMLElement("iframe") }}. This is a security measure. Details as to why this matters are forthcoming. See {{ bug(595307) }}.</p>
<h2 id="Next_step">Next step</h2>
<p>If you want to start tinkering with the API, jump in to the <a href="/en/IndexedDB" title="https://developer.mozilla.org/en/IndexedDB">reference documentation</a> and checking out the different methods.</p>
<h2 id="See_also">See also</h2>
<p>Reference</p>
<ul>
  <li><a href="/en/IndexedDB" title="https://developer.mozilla.org/en/IndexedDB">IndexedDB&nbsp;API&nbsp;Reference</a></li>
  <li><a class="external" href="http://www.w3.org/TR/IndexedDB/" title="http://www.w3.org/TR/IndexedDB/">Indexed Database API Specification</a></li>
</ul>
<p>Tutorials</p>
<ul>
  <li><a class="external" href="http://www.html5rocks.com/tutorials/indexeddb/todo/" title="http://www.html5rocks.com/tutorials/indexeddb/todo/">A simple TODO&nbsp;list using HTML5 IndexedDB</a><span class="external">. {{Note("This tutorial is based on an old version of the specification and does not work on up-to-date browsers - it still uses the removed <code>setVersion()</code> method.") }}</span></li>
  <li><a href="http://www.html5rocks.com/en/tutorials/indexeddb/uidatabinding/" title="http://www.html5rocks.com/en/tutorials/indexeddb/uidatabinding/">Databinding UI Elements with IndexedDB</a></li>
</ul>
<p>Related articles</p>
<ul>
  <li><a class="external" href="http://msdn.microsoft.com/en-us/scriptjunkie/gg679063.aspx" title="http://msdn.microsoft.com/en-us/scriptjunkie/gg679063.aspx">IndexedDB&nbsp;— The Store in Your Browser</a></li>
</ul>
<p>Firefox</p>
<ul>
  <li>Mozilla <a class="link-https" href="https://mxr.mozilla.org/mozilla-central/find?text=&amp;string=dom%2FindexedDB%2F.*%5C.idl&amp;regexp=1" title="https://mxr.mozilla.org/mozilla-central/find?text=&amp;string=dom/indexedDB/.*\.idl&amp;regexp=1">interface files</a></li>
  <li><a href="/en/IndexedDB" title="https://developer.mozilla.org/en/IndexedDB">Using JavaScript Generators in Firefox&nbsp;</a></li>
</ul>
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