Deep-copy an object, similar to calling JSON.parse(JSON.stringify(obj)) but preserves dates and undefined

clone.js

function clone(obj) {
  if (Object(obj) !== obj) return obj;
  if (typeof obj.toJSON == 'function') {
    return obj.toJSON();
  }
  var type = toString.call(obj).slice(8, -1);
  if (type in CLONE) {
    return CLONE[type].call(obj, clone);
  }
  var copy = {};
  var keys = Object.keys(obj);
  for (var i = 0, len = keys.length; i < len; i++) {
    var key = keys[i];
    copy[key] = clone(obj[key]);
  }
  return copy;
}

var CLONE = {
  'Array': function(clone) {
    return Array.prototype.map.call(this, clone);
  },
  'Date': function() {
    return new Date(this.valueOf());
  },
  'String': String.prototype.valueOf,
  'Number': Number.prototype.valueOf,
  'Boolean': Boolean.prototype.valueOf
};