transition.select()

transition.select(selector)

For each selected element, selects the first descendant element that matches the specified selector string, if any, and returns a transition on the resulting selection. The selector may be specified either as a selector string or a function. If a function, it is evaluated for each selected element, in order, being passed the current datum d and index i, with the this context as the current DOM element. The new transition has the same id, name and timing as this transition; however, if a transition with the same id already exists on a selected element, the existing transition is returned for that element.

This method is equivalent to deriving the selection for this transition via transition.selection, creating a subselection via selection.select, and then creating a new transition via selection.transition:

transition
  .selection()
  .select(selector)
  .transition(transition)