js报错 style未定义...tab选项卡

　

js报错 style未定义...tab选项卡js代码:
function taba(){
var odiv=document.getElementById('tab1_div');
var oul=document.getElementById('tab1_bton');

var oli=oul.getElementsByTagName('li');
var ocon=odiv.getElementsByTagName('ul')[1];

for(i=0;i oli[i].index=i;
oli[i].onclick=function(){
ocon[i].style.display='block';
}
ocon[this.index].style.display='none';
}

}
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html结构代码:

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报错:Uncaught TypeError: Cannot read property 'style' of undefined
oli.(anonymous function).onclick