Configuring a Safe Rollback for jQuery After a Failed Async Load

Goal: Ensure that after an asynchronous attempt to load a newer jQuery version fails, the application can continue operating with the previously loaded version without breaking plugins or ready handlers.

Constraint: jQuery provides no built‑in rollback API, so developers must manually preserve a reference to the old version before the upgrade attempt. Additionally, the timing of the ready event and plugin bindings can create a race condition if a partially loaded version temporarily overwrites the $ or jQuery reference.

Uncertainty: It is unclear whether simply storing jQuery.fn.jquery before the upgrade is sufficient to detect a failed load, or whether a full copy of the jQuery object is needed to guarantee that existing plugins remain bound to the original version. Moreover, there is no documented pattern for safely re‑invoking ready callbacks after a failed upgrade.

Questions: What is the recommended way to preserve a functional copy of jQuery before attempting an async upgrade? How can developers ensure that ready handlers and plugin bindings are not transferred to a partially loaded version? Is checking jQuery.fn.jquery after the onerror event enough to confirm a failed load, or are additional checks required?