When using a key function to join data and "immutable" data structures, which completely renew the whole dataset after some modification, D3 assigned event handlers keep a reference to the old data in the __on property.

When removing elements via exit(), detached SVG elements remain in memory by the scope present in __on, causing a memory leak. Although these leaks can be prevented by reassigning the event handlers for existing elements as well (e.g. after merge) it would be more convenient and less error prone, when the only data binding would be through selection.data.