Function extend

lib/parallel.js:14–20

(from, to)

  const _supports = !!(isNode || self.Worker); // node always supports parallel

  function extend(from, to) {
    if (!to) to = {};
    for (const i in from) {
      if (to[i] === undefined) to[i] = from[i];
    }
    return to;
  }

  function Operation() {
    this._callbacks = [];