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Showing content from http://developer.mozilla.org/en-US/docs/Web/API/IDBObjectStore/add below:

IDBObjectStore: add() method - Web APIs

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Note: This feature is available in Web Workers.

The add() method of the IDBObjectStore interface returns an IDBRequest object, and, in a separate thread, creates a structured clone of the value, and stores the cloned value in the object store. This is for adding new records to an object store.

To determine if the add operation has completed successfully, listen for the transaction's complete event in addition to the IDBObjectStore.add request's success event, because the transaction may still fail after the success event fires. In other words, the success event is only triggered when the transaction has been successfully queued.

The add method is an insert only method. If a record already exists in the object store with the key parameter as its key, then an error ConstraintError event is fired on the returned request object. For updating existing records, you should use the IDBObjectStore.put method instead.

add(value)
add(value, key)

value

The value to be stored.

key Optional

The key to use to identify the record. If unspecified, it results to null.

An IDBRequest object on which subsequent events related to this operation are fired.

If the operation is successful, the value of the request's result property is the key for the new record.

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

ReadOnlyError DOMException

Thrown if the transaction associated with this operation is in read-only mode.

TransactionInactiveError DOMException

Thrown if this IDBObjectStore's transaction is inactive.

DataError DOMException

Thrown if any of the following conditions apply:

• The object store uses in-line keys or has a key generator, and a key parameter was provided.
• The object store uses out-of-line keys and has no key generator, and no key parameter was provided.
• The object store uses in-line keys but no key generator, and the object store's key path does not yield a valid key.
• The key parameter was provided but does not contain a valid key.

InvalidStateError DOMException

Thrown if the IDBObjectStore has been deleted or removed.

DataCloneError DOMException

Thrown if the data being stored could not be cloned by the internal structured cloning algorithm.

ConstraintError DOMException

Thrown if an insert operation failed because the primary key constraint was violated (due to an already existing record with the same primary key value).

In the following code snippet, we open a read/write transaction on our database and add some data to an object store using add(). Note also the functions attached to transaction event handlers to report on the outcome of the transaction opening in the event of success or failure. For a full working example, see our To-do Notifications app (view example live).

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

DBOpenRequest.onsuccess = (event) => {
  note.appendChild(document.createElement("li")).textContent =
    "Database initialized.";

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

  // Run the addData() function to add the data to the database
  addData();
};

function addData() {
  // Create a new object ready to insert into the IDB
  const newItem = [
    {
      taskTitle: "Walk dog",
      hours: 19,
      minutes: 30,
      day: 24,
      month: "December",
      year: 2013,
      notified: "no",
    },
  ];

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

  // report on the success of the transaction completing, when everything is done
  transaction.oncomplete = (event) => {
    note.appendChild(document.createElement("li")).textContent =
      "Transaction completed.";
  };

  transaction.onerror = (event) => {
    note.appendChild(document.createElement("li")).textContent =
      "Transaction not opened due to error. Duplicate items not allowed.";
  };

  // create an object store on the transaction
  const objectStore = transaction.objectStore("toDoList");

  // Make a request to add our newItem object to the object store
  const objectStoreRequest = objectStore.add(newItem[0]);

  objectStoreRequest.onsuccess = (event) => {
    // report the success of our request
    note.appendChild(document.createElement("li")).textContent =
      "Request successful.";
  };
}

• 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).

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