extend时添加undefined判断
extend时添加undefined判断

thinkjs/thinkjs,akira-cn/thinkjs,75team/thinkjs,maxzhang/thinkjs,74sharlock/thinkjs,maxzhang/thinkjs,74sharlock/thinkjs,75team/thinkjs,akira-cn/thinkjs,Qihoo360/thinkjs,maxzhang/thinkjs,snadn/thinkjs,akira-cn/thinkjs,lukw00/thinkjs,75team/thinkjs,lukw00/thinkjs