d3.transition()

d3.transition([name])

Returns a new transition on the root element, document.documentElement, with the specified name. If a name is not specified, null is used. The new transition is only exclusive with other transitions of the same name. The name may also be a transition instance; see selection.transition. This method is equivalent to:

d3.selection()
  .transition(name)

This function can also be used to test for transitions (instanceof d3.transition) or to extend the transition prototype.