source: node_modules/d3-transition/src/selection/transition.js

import {Transition, newId} from "../transition/index.js";
import schedule from "../transition/schedule.js";
import {easeCubicInOut} from "d3-ease";
import {now} from "d3-timer";

var defaultTiming = {
  time: null, // Set on use.
  delay: 0,
  duration: 250,
  ease: easeCubicInOut
};

function inherit(node, id) {
  var timing;
  while (!(timing = node.__transition) || !(timing = timing[id])) {
    if (!(node = node.parentNode)) {
      throw new Error(`transition ${id} not found`);
    }
  }
  return timing;
}

export default function(name) {
  var id,
      timing;

  if (name instanceof Transition) {
    id = name._id, name = name._name;
  } else {
    id = newId(), (timing = defaultTiming).time = now(), name = name == null ? null : name + "";
  }

  for (var groups = this._groups, m = groups.length, j = 0; j < m; ++j) {
    for (var group = groups[j], n = group.length, node, i = 0; i < n; ++i) {
      if (node = group[i]) {
        schedule(node, name, id, i, group, timing || inherit(node, id));
      }
    }
  }

  return new Transition(groups, this._parents, name, id);
}