指定したIDの要素を取得
element = document.getElementById(id);
<style>
#geteleid_div01 {
border:1px solid;
width:100px;
margin-bottom:10px;
padding:10px;
}
</style>
<script>
function func_geteleid(chawide) {
ele=document.getElementById('geteleid_div01');
ele.style.width = chawide;
}
</script>
<div class="geteleid01">
<div id="geteleid_div01">要素1</div>
<button onclick="func_geteleid('300px');">幅を広げる</button>
<button onclick="func_geteleid('100px');">幅を戻す</button>
</div>
