The createIndex() method of the IDBObjectStore interface creates and returns a new IDBIndex object in the connected database. It creates a new field/column defining a new data point for each database record to contain.

Bear in mind that IndexedDB indexes can contain any JavaScript data type; IndexedDB uses the structured clone algorithm to serialize stored objects, which allows for storage of simple and complex objects.

Note that this method must be called only from a VersionChange transaction mode callback.

indexName

The name of the index to create. Note that it is possible to create an index with an empty name.

keyPath

The key path for the index to use. Note that it is possible to create an index with an empty keyPath, and also to pass in a sequence (array) as a keyPath.

options Optional

An object which can include the following properties:

unique

If true, the index will not allow duplicate values for a single key. Defaults to false.

multiEntry

If true, the index will add an entry in the index for each array element when the keyPath resolves to an array. If false, it will add one single entry containing the array. Defaults to false.

locale Non-standard

Currently Firefox-only (43+), this allows you to specify a locale for the index. Any sorting operations performed on the data via key ranges will then obey sorting rules of that locale (see locale-aware sorting). You can specify its value in one of three ways:

• string: A string containing a specific locale code, e.g. en-US, or pl.
• auto: The platform default locale will be used (may be changed by user agent settings).
• null or undefined: If no locale is specified, normal JavaScript sorting will be used — not locale-aware.

An IDBIndex object: the newly created index.

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

ConstraintError DOMException

Thrown if an index with the same name already exists in the database. Index names are case-sensitive.

InvalidAccessError DOMException

Thrown if the provided key path is a sequence, and multiEntry is set to true in the objectParameters object.

InvalidStateError DOMException

Thrown if:

• The method was not called from a versionchange transaction mode callback, i.e. from inside a onupgradeneeded handler.
• The object store has been deleted.

SyntaxError DOMException

Thrown if the provided keyPath is not a valid key path.

TransactionInactiveError DOMException

Thrown if the transaction this IDBObjectStore belongs to is not active (e.g. has been deleted or removed.) In Firefox previous to version 41, an InvalidStateError was raised in this case as well, which was misleading; this has now been fixed (see Webkit bug 1176165.)

In the following example you can see the onupgradeneeded handler being used to update the database structure if a database with a higher version number is loaded. createIndex() is used to create new indexes on the object store. For a full working example, see our To-do Notifications app (view example live).

let db;

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

// Two event handlers for opening the database.
DBOpenRequest.onerror = (event) => {
  note.innerHTML += "<li>Error loading database.</li>";
};

DBOpenRequest.onsuccess = (event) => {
  note.innerHTML += "<li>Database initialized.</li>";

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

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

// This handler fires when a new database is created and indicates
// either that one has not been created before, or a new version
// was submitted with window.indexedDB.open(). (See above.)
// It is only implemented in recent browsers.
DBOpenRequest.onupgradeneeded = (event) => {
  const db = event.target.result;

  db.onerror = (event) => {
    note.innerHTML += "<li>Error loading database.</li>";
  };

  // Create an objectStore for this database
  const objectStore = db.createObjectStore("toDoList", {
    keyPath: "taskTitle",
  });

  // define what data items the objectStore will contain

  objectStore.createIndex("hours", "hours", { unique: false });
  objectStore.createIndex("minutes", "minutes", { unique: false });
  objectStore.createIndex("day", "day", { unique: false });
  objectStore.createIndex("month", "month", { unique: false });
  objectStore.createIndex("year", "year", { unique: false });
  objectStore.createIndex("notified", "notified", { unique: false });
};

• 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 the example live).