js中Cannot read property 'style' of undefined

Question

<script> window.onload=function(){     var div=document.getElementById('content')     var btn=div.getElementsByTagName('input')     var div_d=div.getElementsByTagName('div')          for (var i = 0; i < btn.length; i++) {         btn[i].onclick=function () {             for (var i2 = 0; i2 < div_d.length; i2++) {                 div_d[i2].style.display='none'             }             div_d[i].style.display='block'//这行报错         }     } } </script>

Answer 1

 你button的个数和div的个数是一样的？