src/selection/node.js

import "selection";

d3_selectionPrototype.node = function() {
  for (var j = 0, m = this.length; j < m; j++) {
    for (var group = this[j], i = 0, n = group.length; i < n; i++) {
      var node = group[i];
      if (node) return node;
    }
  }
  return null;
};