assign = function(object, source, guard) {
      var index, iterable = object, result = iterable;
      if (!iterable) return result;
      var args = arguments,
          argsIndex = 0,
          argsLength = typeof guard == 'number' ? 2 : args.length;
      if (argsLength > 3 && typeof args[argsLength - 2] == 'function') {
        var callback = baseCreateCallback(args[--argsLength - 1], args[argsLength--], 2);
      } else if (argsLength > 2 && typeof args[argsLength - 1] == 'function') {
        callback = args[--argsLength];
      }
      while (++argsIndex < argsLength) {
        iterable = args[argsIndex];
        if (iterable && objectTypes[typeof iterable]) {
        var ownIndex = -1,
            ownProps = objectTypes[typeof iterable] && keys(iterable),
            length = ownProps ? ownProps.length : 0;

        while (++ownIndex < length) {
          index = ownProps[ownIndex];
          result[index] = callback ? callback(result[index], iterable[index]) : iterable[index];
        }
        }
      }
      return result
    }
Assigns own enumerable properties of source object(s) to the destination
object. Subsequent sources will overwrite property assignments of previous
sources. If a callback is provided it will be executed to produce the
assigned values. The callback is bound to `thisArg` and invoked with two
arguments; (objectValue, sourceValue).

@static
@memberOf _
@type Function
@alias extend
@category Objects
@param {Object} object The destination object.
@param {...Object} [source] The source objects.
@param {Function} [callback] The function to customize assigning values.
@param {*} [thisArg] The `this` binding of `callback`.
@returns {Object} Returns the destination object.
@example

_.assign({ 'name': 'fred' }, { 'employer': 'slate' });
// => { 'name': 'fred', 'employer': 'slate' }

var defaults = _.partialRight(_.assign, function(a, b) {
  return typeof a == 'undefined' ? b : a;
});

var object = { 'name': 'barney' };
defaults(object, { 'name': 'fred', 'employer': 'slate' });
// => { 'name': 'barney', 'employer': 'slate' }