JSFiddle - React, Tailwind, and code Playground

HTML

<div id="holder">
  <div class="menu_item">
    item1
  </div>
  <div class="menu_item">
    item2
  </div>
  <div class="menu_item">
    item3
  </div>
</div>

<div id="output"></div>

CSS

.holder {
    width:100%; /* which in this case is 320px */
    overflow-x: auto;
    overflow-y:hidden;
  }

  .menu_item {
    width:200px;
    float:left;
  }

JavaScript

var output = document.getElementById("output");

output.innerHTML = "Width: " + document.getElementById("holder").clientWidth;