src/transition/filter.js

import "../selection/filter";
import "transition";

d3_transitionPrototype.filter = function(filter) {
  var subgroups = [],
      subgroup,
      group,
      node;

  if (typeof filter !== "function") filter = d3_selection_filter(filter);

  for (var j = 0, m = this.length; j < m; j++) {
    subgroups.push(subgroup = []);
    for (var group = this[j], i = 0, n = group.length; i < n; i++) {
      if ((node = group[i]) && filter.call(node, node.__data__, i, j)) {
        subgroup.push(node);
      }
    }
  }

  return d3_transition(subgroups, this.id);
};