A function for triggering a callback at the end of D3.js transitions

onD3TransitionsEnd.js

/*
  Accepts one or more transitions and a callback as last argument
  Example: 
  ```
    var transition1 = d3.selectAll('g').transition().duration(500)
      , transition2 = d3.selectAll('circle').transition().duration(400)
      , callback = function() {console.log('done')}
      
    onD3TransitionsEnd(transition1, transition2, callback)
  ```
*/
function onD3TransitionsEnd() {
  var args = Array.prototype.slice.call(arguments)
    , selectionCount = 0
    , cb = args[args.length - 1]

  for (var i = 0; i < args.length - 1; i++) {
    selectionCount += args[i].length
    args[i].each('end', function(){
      selectionCount -= 1
      selectionCount === 0 && cb()
    })
  }
}