code: function union(arr1, arr2) {
    if (_.isEqual(arr1, arr2)) {
      return arr1;
    }
  
    const arr = arr1.slice();
    for (const v of arr2) {
      if (!arrayIncludes(arr, v)) {
        arr.push(v);
      }
    }
  
    return arr;
  }
docs: _.union doesn't work with ObjectIds, TODO: replace with _.unionWith(..., Tyr.isEqual) when lodash upgraded