Yet another jquery-like deep extend implementation

extend.js

module.exports = (function () {

  var extend_single = function (destination, source, deep) {

    for (var key in source) {
      if (source.hasOwnProperty(key)) {
        if (deep && typeof destination[key] == 'object' && typeof source[key] == 'object') {
          destination[key] = extend_single(destination[key], source[key], deep);
        } else {
          destination[key] = source[key];
        }
      }
    }

    return destination;
  }

  var extend = function (is_deep) {

    deep_extend = (is_deep && typeof deep_extend !== 'object') ? (is_deep === null ? false : true) : false;
    var start = (typeof is_deep === 'object' && is_deep != null) ? 0 : 1;
    if (arguments.length - start <= 0) return {};
    var to = arguments[start];

    for (var index = start + 1; index < arguments.length; index++) {
      to = extend_single(to, arguments[index], deep_extend);
    }
    
    return to;
  }

  return extend;

})();