indexKeys = function indexKeys(target, pred) {
  pred = pred || has;
  var results = [];

  for (var i = 0, len = target.length; i < len; i += 1) {
    if (pred(target, i)) {
      results.push(String(i));
    }
  }

  return results;
}
indexKeys

@name indexKeys
@api private
@param {} target
@param {} pred
@return {Array}