function mergeDeep(...objects) {

  // For simplicity, we ignore the cases for Date and Regex
  const isObject = obj => obj && typeof obj === 'object';

  return objects.reduce((prev, obj) => {
    try {
      Object.keys(obj).forEach(key => {
        const pVal = prev[key];
        const oVal = obj[key];

        if (Array.isArray(pVal) && Array.isArray(oVal)) {
          prev[key] = pVal.concat(...oVal);
        } else if (isObject(pVal) && isObject(oVal)) {
          prev[key] = mergeDeep(pVal, oVal);
        } else {
          prev[key] = oVal;
        }
      });

      return prev;
    } catch (rangeError) {
      if (rangeError instanceof RangeError) {
        console.log(rangeError.message);
      }
      return prev;
    }
  }, {});
}

mergeDeep({a: 1, b: 2}, {c: 3, d: 4}); // {a: 1, b: 2, c: 3, d: 4}
mergeDeep([{a: 1, b: 2}], [{c: 3, d: 4}]); // [{a: 1, b: 2, c: 3, d: 4}]

var object = {
  y: 2,
};
var other = {};
object.x = other;
other.y = object;
mergeDeep(object, other); // {y: {y: 2, x: {...}}, x: {y: {...}}}