function clone(parent, circular, depth, prototype) {
  var filter;
  if (typeof circular === 'object') {
    depth = circular.depth;
    prototype = circular.prototype;
    filter = circular.filter;
    circular = circular.circular
  }
  // maintain two arrays for circular references, where corresponding parents
  // and children have the same index
  var allParents = [];
  var allChildren = [];

  var useBuffer = typeof Buffer != 'undefined';

  if (typeof circular == 'undefined')
    circular = true;

  if (typeof depth == 'undefined')
    depth = Infinity;

  // recurse this function so we don't reset allParents and allChildren
  function _clone(parent, depth) {
    // cloning null always returns null
    if (parent === null)
      return null;

    if (depth == 0)
      return parent;

    var child;
    var proto;
    if (typeof parent != 'object') {
      return parent;
    }

    if (clone.__isArray(parent)) {
      child = [];
    } else if (clone.__isRegExp(parent)) {
      child = new RegExp(parent.source, __getRegExpFlags(parent));
      if (parent.lastIndex) child.lastIndex = parent.lastIndex;
    } else if (clone.__isDate(parent)) {
      child = new Date(parent.getTime());
    } else if (useBuffer && Buffer.isBuffer(parent)) {
      if (Buffer.allocUnsafe) {
        // Node.js >= 4.5.0
        child = Buffer.allocUnsafe(parent.length);
      } else {
        // Older Node.js versions
        child = new Buffer(parent.length);
      }
      parent.copy(child);
      return child;
    } else {
      if (typeof prototype == 'undefined') {
        proto = Object.getPrototypeOf(parent);
        child = Object.create(proto);
      }
      else {
        child = Object.create(prototype);
        proto = prototype;
      }
    }

    if (circular) {
      var index = allParents.indexOf(parent);

      if (index != -1) {
        return allChildren[index];
      }
      allParents.push(parent);
      allChildren.push(child);
    }

    for (var i in parent) {
      var attrs;
      if (proto) {
        attrs = Object.getOwnPropertyDescriptor(proto, i);
      }

      if (attrs && attrs.set == null) {
        continue;
      }
      child[i] = _clone(parent[i], depth - 1);
    }

    return child;
  }

  return _clone(parent, depth);
}
Clones (copies) an Object using deep copying.

This function supports circular references by default, but if you are certain
there are no circular references in your object, you can save some CPU time
by calling clone(obj, false).

Caution: if `circular` is false and `parent` contains circular references,
your program may enter an infinite loop and crash.

@param `parent` - the object to be cloned
@param `circular` - set to true if the object to be cloned may contain
   circular references. (optional - true by default)
@param `depth` - set to a number if the object is only to be cloned to
   a particular depth. (optional - defaults to Infinity)
@param `prototype` - sets the prototype to be used when cloning an object.
   (optional - defaults to parent prototype).