<div class="button"><div>
<div class="class-main">
<div class="class">1</div>
<div class="class">2</div>
<div class="class">3</div>
</div>
<div class="button">Button</div>
<div class="class-main">
<div class="class">1</div>
<div class="class">2</div>
<div class="class">3</div>
</div>
<script type="text/javascript">
document.getElementsByClassName("button")[0].onclick=function(){
console.log(document.getElementsByClassName("class")[1])
}
</script>