console
var btn=document.getElementById('btn')
var btn1=document.getElementById('reset')
var tx=document.getElementsByTagName('span')
var content=document.getElementById('content')
var arr=[];
var n=0;
btn.onclick=function(){7
var x=Math.floor(Math.random()*(49-1+1)+1)
x=PrefixZero(x,2);
content.innerHTML+='<span>'+x+'</span>'
n++;
if(n==7){
this.disabled=true;
tx[n-1].className='active'
}
console.log(tx.length)
}
btn1.onclick=function(){
content.innerHTML="";
btn.disabled=false;
n=0;
}
function PrefixZero(num, n) {
return (Array(n).join(0) + num).slice(-n);
}
<input id="btn" type="button" value="开始" />
<input id="reset" type="button" value="重置" />
<input id="third" type="button" value="三" />
</br> </br> </br>
<div id="content">
</div>
span{
width:30px;
height:30px;
line-height:30px;
display:block;
background:skyblue;
text-align: center;
margin:10px;
border-radius: 50%;
float:left;
}
.active{
background:red;
}