IDBDatabase.transaction() - Web APIs 编辑

The transaction method of the IDBDatabase interface immediately returns a transaction object (IDBTransaction) containing the IDBTransaction.objectStore method, which you can use to access your object store.

Note:

Syntax

IDBDatabase.transaction(storeNames);
IDBDatabase.transaction(storeNames, mode);
IDBDatabase.transaction(storeNames, mode, options);

Parameters

storeNames

The names of object stores that are in the scope of the new transaction, declared as an array of strings. Specify only the object stores that you need to access.
If you need to access only one object store, you can specify its name as a string. Therefore the following lines are equivalent:

var transaction = db.transaction(['my-store-name']);
var transaction = db.transaction('my-store-name');

If you need to access all object stores in the database, you can use the property IDBDatabase.objectStoreNames:

var transaction = db.transaction(db.objectStoreNames);

Passing an empty array will throw an exception.

mode Optional

The types of access that can be performed in the transaction. Transactions are opened in one of three modes: readonly, readwrite and readwriteflush (non-standard, Firefox-only.) versionchange mode can't be specified here. If you don't provide the parameter, the default access mode is readonly. To avoid slowing things down, don't open a readwrite transaction unless you actually need to write into the database.

If you need to open the object store in readwrite mode to change data, you would use the following:

var transaction = db.transaction('my-store-name', "readwrite");

As of Firefox 40, IndexedDB transactions have relaxed durability guarantees to increase performance (see bug 1112702), which is the same behaviour as other IndexedDB-supporting browsers. Previously in a readwrite transaction IDBTransaction.oncomplete was fired only when all data was guaranteed to have been flushed to disk. In Firefox 40+ the complete event is fired after the OS has been told to write the data but potentially before that data has actually been flushed to disk. The complete event may thus be delivered quicker than before, however, there exists a small chance that the entire transaction will be lost if the OS crashes or there is a loss of system power before the data is flushed to disk. Since such catastrophic events are rare most consumers should not need to concern themselves further.

Note: In Firefox, if you wish to ensure durability for some reason (e.g. you're storing critical data that cannot be recomputed later) you can force a transaction to flush to disk before delivering the complete event by creating a transaction using the experimental (non-standard) readwriteflush mode (see IDBDatabase.transaction.) This is currently experimental, and can only be used if the dom.indexedDB.experimental pref is set to true in about:config.

options Optional

Dictionary of other options. Available options are:

• durability: "default", "strict", or "relaxed". The default is "default". Using "relaxed" provides better performance, but with fewer guarantees. Web applications are encouraged to use "relaxed" for ephemeral data such as caches or quickly changing records, and "strict" in cases where reducing the risk of data loss outweighs the impact to performance and power.

Return value

An IDBTransaction object.

Exceptions

This method may raise a DOMException of one of the following types:

Example

In this example we open a database connection, then use transaction() to open a transaction on the database. For a complete example, see our To-do Notifications app (view example live.)

var db;

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

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

  // store the result of opening the database in the db variable.
  // This is used a lot below
  db = DBOpenRequest.result;

  // Run the displayData() function to populate the task list with
  // all the to-do list data already in the IDB
  displayData();

};

// open a read/write db transaction, ready for adding the data
var transaction = db.transaction(["toDoList"], "readwrite");

// report on the success of opening the transaction
transaction.oncomplete = function(event) {
  note.innerHTML += '<li>Transaction completed: database modification finished.</li>';
};

transaction.onerror = function(event) {
  note.innerHTML += '<li>Transaction not opened due to error. Duplicate items not allowed.</li>';
};

// you would then go on to do something to this database
// via an object store
var objectStore = transaction.objectStore("toDoList");
// etc.

Specification

Browser compatibility

BCD tables only load in the browser

See also

• Using IndexedDB
• Starting transactions: IDBDatabase
• Using transactions: IDBTransaction
• Setting a range of keys: IDBKeyRange
• Retrieving and making changes to your data: IDBObjectStore
• Using cursors: IDBCursor
• Reference example: To-do Notifications (view example live.)