原生JS获取元素集合的子元素宽度实例

<ul class="itemCon">
  <li class="item">
    <div class="leftMess">
      <div class="leftCon">
        <span>头像</span>
        <p>名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字</p>
      </div>
    </div>
    <div class="rightMess">
      <span>工具1</span>
      <span>工具2</span>
      <span>工具3</span>
    </div>
  </li>
  <li class="item">
    <div class="leftMess">
      <div class="leftCon">
        <span>头像</span>
        <p>名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字</p>
      </div>
    </div>
    <div class="rightMess">
      <span>工具2</span>
      <span>工具3</span>
    </div>
  </li>
  <li class="item" id="test">
    <div class="leftMess">
      <div class="leftCon">
        <span>头像</span>
        <p>名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字名字</p>
      </div>
    </div>
    <div class="rightMess">
      <span>工具3</span>
    </div>
  </li>
</ul>

html,body{padding:0;margin:0}
ul,li{list-style:none;padding:0;margin:0}
p{margin:0;padding:0}
.itemCon{width:450px;height:auto; margin:100px auto; background:#c; overflow:hidden}
.item{width:100%; float:left;height:60px;border-bottom:solid 1px #f00; position:relative}
.leftMess{ height:60px;}
.leftCon{ float:left; height:60px; overflow:hidden;width:100%}
.leftMess span{ display:inline-block;width:50px;height:50px; border-radius:50%; background:#f00; text-align:center; line-height:50px; margin:5px; float:left}
.leftMess p{ margin-left:60px}
.rightMess{width:auto;height:60px; position:absolute;right:0;:0}
.rightMess span{width:50px; height:50px; line-height:50px; text-align:center; margin:5px; float:left; background:#f00}

<script type="text/javascript">
  var divs = document.getElementsByClassName('item');
  for (var i = 0; i < divs.length; i++) {
    var lastW = divs[i].children[1].offsetWidth;
    divs[i].children[0].style.marginRight=lastW+"px"
  }
</script>