What going from ES5 to ES6 can look like

generateBoilerplatedKeyframes.es5.js

/*!
 * @param {Rekapi.Actor} actor
 * @param {string} animName
 * @param {number} steps
 * @param {boolean} doCombineProperties
 * @param {Array.<string>=} opt_vendors
 * @return {string}
 */
function generateBoilerplatedKeyframes (
    actor, animName, steps, doCombineProperties, opt_vendors) {

  var trackNames = actor.getTrackNames();
  var cssTracks = [];

  if (doCombineProperties) {
    cssTracks.push(generateCombinedActorKeyframes(actor, steps));
  } else {
    _.each(trackNames, function (trackName) {
      cssTracks.push(
        generateActorKeyframes(actor, steps, trackName));
    });
  }

  var boilerplatedKeyframes = [];

  if (doCombineProperties) {
    boilerplatedKeyframes.push(applyVendorBoilerplates(
      cssTracks[0], (animName), opt_vendors));
  } else {
    _.each(trackNames, function (trackName, i) {
      boilerplatedKeyframes.push(applyVendorBoilerplates(
        cssTracks[i], (animName + '-' + trackName), opt_vendors));
    });
  }

  boilerplatedKeyframes = boilerplatedKeyframes.join('\n');

  return boilerplatedKeyframes;
}

generateBoilerplatedKeyframes.es6.js

/*!
 * @param {Rekapi.Actor} actor
 * @param {string} animName
 * @param {number} steps
 * @param {boolean} doCombineProperties
 * @param {Array.<string>=} vendors
 * @return {string}
 */
const generateBoilerplatedKeyframes = (
  actor, animName, steps, doCombineProperties, vendors = undefined) =>

  doCombineProperties ?
    applyVendorBoilerplates(
      generateCombinedActorKeyframes(actor, steps),
      animName,
      vendors
    ) :
    actor.getTrackNames().map(trackName =>
      applyVendorBoilerplates(
        generateActorKeyframes(actor, steps, trackName),
        `${animName}-${trackName}`,
        vendors
      )
    ).join('\n');