The undefined type in the client does not have an explicit corresponding type, it will resulting from the `Object` to `String`, this problem will cause the iOS application to crash.
JS Framework source code: https://github.com/alibaba/weex/blame/7a9bb149f415f0cf49e4e5f59bf743f7671e3688/src/js-framework/lib/util/index.js#L151 Dotwe demo: http://dotwe.org/vue/94e3d5e3f7f9017c49d82bcfb927f879 Thanks.
