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

import {get, set} from "./schedule.js";

function tweenRemove(id, name) {
  var tween0, tween1;
  return function() {
    var schedule = set(this, id),
        tween = schedule.tween;

    // If this node shared tween with the previous node,
    // just assign the updated shared tween and we’re done!
    // Otherwise, copy-on-write.
    if (tween !== tween0) {
      tween1 = tween0 = tween;
      for (var i = 0, n = tween1.length; i < n; ++i) {
        if (tween1[i].name === name) {
          tween1 = tween1.slice();
          tween1.splice(i, 1);
          break;
        }
      }
    }

    schedule.tween = tween1;
  };
}

function tweenFunction(id, name, value) {
  var tween0, tween1;
  if (typeof value !== "function") throw new Error;
  return function() {
    var schedule = set(this, id),
        tween = schedule.tween;

    // If this node shared tween with the previous node,
    // just assign the updated shared tween and we’re done!
    // Otherwise, copy-on-write.
    if (tween !== tween0) {
      tween1 = (tween0 = tween).slice();
      for (var t = {name: name, value: value}, i = 0, n = tween1.length; i < n; ++i) {
        if (tween1[i].name === name) {
          tween1[i] = t;
          break;
        }
      }
      if (i === n) tween1.push(t);
    }

    schedule.tween = tween1;
  };
}

export default function(name, value) {
  var id = this._id;

  name += "";

  if (arguments.length < 2) {
    var tween = get(this.node(), id).tween;
    for (var i = 0, n = tween.length, t; i < n; ++i) {
      if ((t = tween[i]).name === name) {
        return t.value;
      }
    }
    return null;
  }

  return this.each((value == null ? tweenRemove : tweenFunction)(id, name, value));
}

export function tweenValue(transition, name, value) {
  var id = transition._id;

  transition.each(function() {
    var schedule = set(this, id);
    (schedule.value || (schedule.value = {}))[name] = value.apply(this, arguments);
  });

  return function(node) {
    return get(node, id).value[name];
  };
}