FWIW, the underlying jQuery code explicitly tests for false with ===: https://github.com/jquery/jquery/blob/a5037cb9e3851b171b49f6d717fb40e59aa344c2/src/event.

FWIW, the underlying jQuery code explicitly tests for false with ===: https://github.com/jquery/jquery/blob/a5037cb9e3851b171b49f6d717fb40e59aa344c2/src/event.js#L317 https://github.com/jquery/jquery/blob/a5037cb9e3851b171b49f6... So returning undefined does NOT stop propagation.