Called if the promise wasn’t caught.

Deprecated. This event is deprecated and obsolete since 4.x

Sample

var p = new Promise(function (resolve, reject) {
    reject("Failed!");
});
p.onuncatched = function (e) {
    alert ("No-one caught this error: " + e);
}

Result:

Since neither then(x, f), catch(f) or finally(f) was called with a valid function f, the alert would be fired.

How this is used in Dexie

Dexie uses this event on all returned promises from any DB operation on a Transaction in order to make sure error is propagated to the transaction and that transaction is aborted.