--- title: IDBIndex slug: Web/API/IDBIndex translation_of: Web/API/IDBIndex ---

{{APIRef()}}

IndexedDB API 中的IDBIndex接口提供了异步获取数据库中一个index的功能。index是一种用于在另一个object store中查找记录的object store，其被称为被引用的object store。你可以通过使用该接口来取回数据。

你可以通过记录的键或使用一个index取回一个object store中的这些记录 (cursors 提供了第三种方式：请见 {{ domxref("IDBCursor") }})。一个index可以让你在object store的records中，通过使用records的properties（属性）来寻找records。

index是一个持久的键-值存储，其中其记录的值部分是被引用object store中的record的关键部分。在object store中新增、更新或是删除records时，索引中的records将自动填充。索引中的每条记录只能指向其引用的object  store中的唯一一条记录，但是多个索引可以引用同一个object store。当object store变更时，所有引用object store的索引都会自动更新。

索引中的records总是按照records的key进行排序。然而，不像object stores，一个给定的index可以包含具有相同key的多条记录。这些records将根据被引用object store中的主键进一步排序。

你可以设置在一个范围内的key，点击这里查看更多： {{domxref("IDBKeyRange")}}.

Methods

Inherits from: EventTarget

{{domxref("IDBIndex.count")}}
Returns an {{domxref("IDBRequest")}} object, and in a separate thread, returns the number of records within a key range.
{{domxref("IDBIndex.get")}}
Returns an {{domxref("IDBRequest")}} object, and, in a separate thread, finds either the value in the referenced object store that corresponds to the given key or the first corresponding value, if key is a key range.
{{domxref("IDBIndex.getAll")}} {{ Non-Standard_inline() }}
Instantly retrieves all objects inside an {{domxref("IDBObjectStore")}}, setting them as the result of the request object.
{{domxref("IDBIndex.getKey")}}
Returns an {{domxref("IDBRequest")}} object, and, in a separate thread, finds either the given key or the primary key, if key is a key range.
{{domxref("IDBIndex.getAllKeys")}} {{ Non-Standard_inline() }}
Instantly retrieves the keys of all objects inside an {{domxref("IDBObjectStore")}}, setting them as the result of the request object.
{{domxref("IDBIndex.openCursor")}}
Returns an {{domxref("IDBRequest")}} object, and, in a separate thread, creates a cursor over the specified key range.
{{domxref("IDBIndex.openKeyCursor")}} {{ Non-Standard_inline() }}
Returns an {{domxref("IDBRequest")}} object, and, in a separate thread, creates a cursor over the specified key range, as arranged by this index.

Properties

{{domxref("IDBIndex.name")}} {{readonlyInline}}
index的名称。
{{domxref("IDBIndex.objectStore")}} {{readonlyInline}}
index所指向的object store的名称。
{{domxref("IDBIndex.keyPath")}} {{readonlyInline}}
此index的关键路径。它如果为null，则index不是自动填充的。
{{domxref("IDBIndex.multiEntry")}} {{readonlyInline}}
影响index的key路径的计算结果生成数组时索引的行为方式。如果为true，那么对于key数组中的每一项，索引中都有一条记录。如果为false，那么对于数组中的每个key都有一条记录。
{{domxref("IDBIndex.unique")}} {{readonlyInline}}
如果为true，这个index不会允许key有重复值。 

Examples

Opening a transaction then using get() to retrieve an object of known key:

// Let us open our database
var request = window.indexedDB.open("toDoList", 4);

// these two event handlers act on the database being opened successfully, or not
request.onerror = function(event) {
  note.innerHTML += '<li>Error loading database.</li>';
};

request.onsuccess = function(event) {
note.innerHTML += '<li>Database initialised.</li>';

// store the result of opening the database in the db variable.
db = request.result;

// Open a transaction on the current database and get a reference to the object store
//that we want to pull information out of
var transaction = db.transaction(["toDoList"]);
var objectStore = transaction.objectStore("toDoList");

// Use get() to get a specific object from the object store, the key of which is "Walk dog"
var request = objectStore.get("Walk dog");
request.onerror = function(event) {
  console.log("There is no record stored for " + request.result.taskTitle);
};
request.onsuccess = function(event) {
  // Do something with the request.result!
  console.log("The deadline time for " + request.result.taskTitle + " is " +
              request.result.hours + ":" + request.result.minutes + ".";
};

Note: need to work out a way to retrieve a series/range of objects using an index, or just all of them. Is this possible with get, or is this a job for cursor?

Specifications

Specification Status Comment
{{SpecName('IndexedDB', '#idl-def-IDBIndex', 'IDBIndex')}} {{Spec2('IndexedDB')}}

Browser compatibility

{{Compat("api.IDBIndex")}}

See also

To learn more about various topics, see the following