jquery ui get id of droppable element, when dropped an item

Answer 1

jQuery UI's droppable API manual mentions how to get the "dropped-on-droppable" very "secretly" in the section of greedy option:

event.target can be checked to see which droppable received the draggable element

But note that event.target contains just a DOM element...

Answer to your question

You will be able to get the ID in your droppable's drop callback through the first parameter event.

Pure JS

Property: event.target.id - if ID is not set: empty string ""
Attribute: event.target.getAttribute('id') - if ID is not set: null

jQuery

Property: $(event.target).prop('id') - if ID is not set: empty string ""
Attribute: $(event.target).attr('id') - if ID is not set: undefined

Usage example

Additional info

Event
jQuery's event system normalizes the event object according to W3C standards.
The event object is guaranteed to be passed to the event handler (no checks for window.event required). It normalizes the target, relatedTarget, which, metaKey and pageX/Y properties and provides both stopPropagation() and preventDefault() methods.