function deepCopy (obj, cache) {
  if ( cache === void 0 ) cache = [];

  // just return if obj is immutable value
  if (obj === null || typeof obj !== 'object') {
    return obj
  }

  // if obj is hit, it is in circular structure
  var hit = find(cache, function (c) { return c.original === obj; });
  if (hit) {
    return hit.copy
  }

  var copy = Array.isArray(obj) ? [] : {};
  // put the copy into cache at first
  // because we want to refer it in recursive deepCopy
  cache.push({
    original: obj,
    copy: copy
  });

  Object.keys(obj).forEach(function (key) {
    copy[key] = deepCopy(obj[key], cache);
  });

  return copy
}
Deep copy the given object considering circular structure.
This function caches all nested objects and its copies.
If it detects circular structure, use cached copy to avoid infinite loop.

@param {*} obj
@param {Array<Object>} cache
@return {*}