Indexeddb: Differences between onsuccess and oncomplete?

Question

I use two different events for the callback to respond when the IndexedDB transaction finishes or is successful:

Let's say... db : IDBDatabase object, tr : IDBTransaction object, os : IDBObjectStore object

tr = db.transaction(os_name,'readwrite');
os = tr.objectStore();

case 1 :

r = os.openCursor();
r.onsuccess = function(){
    if(r.result){
        callback_for_result_fetched();
        r.result.continue;
    }else callback_for_transaction_finish();
}

case 2:

tr.oncomplete = callback_for_transaction_finish();

It is a waste if both of them work similarly. So can you tell me, is there any difference between them?

Answer 1

Sorry for raising up quite an old thread, but it's questioning is a good starting point...

I've looked for a similar question but in a bit different use case and actually found no good answers or even a misleading ones.

Think of a use case when you need to make several writes into the objectStore of even into several ones. You definitely don't want to manage each single write and it's own success and error events. That is the meaning of transaction and this is the (proper) implementation of it for indexedDB:

var trx = dbInstance.transaction([storeIdA, storeIdB], 'readwrite'),
    storeA = trx.objectStore(storeIdA),
    storeB = trx.objectStore(storeIdB);

    trx.oncomplete = function(event) {
        // this code will run only when ALL of the following requests are succeed
        // and only AFTER ALL of them were processed
    };
    trx.onerror = function(error) {
        // this code will run if ANY of the following requests will fail
        // and only AFTER ALL of them were processed
    };

    storeA.put({ key:keyA, value:valueA });
    storeA.put({ key:keyB, value:valueB });
    storeB.put({ key:keyA, value:valueA });
    storeB.put({ key:keyB, value:valueB });

Clue to this understanding is to be found in the following statement of W3C spec:

To determine if a transaction has completed successfully, listen to the transaction’s complete event rather than the success event of a particular request, because the transaction may still fail after the success event fires.

Answer 2

While it's true these callbacks function similarly they are not the same: the difference between onsuccess and oncomplete is that transactions complete but requests, which are made on those transactions, are successful.

oncomplete is only defined in the spec as related to a transaction. A transaction doesn't have an onsuccess callback.