原生JS动态获取右侧元素宽度并设置左侧边距

HTML与CSS结构实现

<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:#ccc; 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;top:0}
.rightMess span{width:50px; height:50px; line-height:50px; text-align:center; margin:5px; float:left; background:#f00}

JavaScript动态获取宽度逻辑

<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>