function defaults(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;
      }
      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];
            if (typeof object[key] == 'undefined') {
              object[key] = source[key];
            }
          }
        }
      }
      return object;
    }
Assigns own enumerable properties of source object(s) to the destination
object for all destination properties that resolve to `undefined`. Once a
property is set, additional defaults of the same property will be ignored.

@static
@memberOf _
@category Objects
@param {Object} object The destination object.
@param {...Object} [source] The source objects.
@param- {Object} [guard] Enables use as a callback for functions like `_.reduce`.
@returns {Object} Returns the destination object.
@example

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