function assign(object, source, guard) {
      var args = arguments,
          argsIndex = 0,
          argsLength = args.length,
          type = typeof guard;

      // enables use as a callback for functions like `_.reduce`
      if ((type == 'number' || type == 'string') && args[3] && args[3][guard] === source) {
        argsLength = 2;
      }
      // juggle arguments
      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) {
        source = args[argsIndex];
        if (isObject(source)) {
          var index = -1,
              props = keys(source),
              length = props.length;

          while (++index < length) {
            var key = props[index];
            object[key] = callback ? callback(object[key], source[key]) : source[key];
          }
        }
      }
      return object;
    }
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 _
@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;
});

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