JSFiddle - React, Tailwind, and code Playground

HTML

<div class="mydiv"></div>

CSS

* {
  box-sizing: inherit;
}

html {
  box-sizing: border-box;
}
body {
  margin: 0;
}

.mydiv {
  width: 300px;
  height: 300px;
  background: #eee;
}

JavaScript

var style = new Array('dotted', 'solid', 'double', 'dashed');
var color = " red";
document.getElementsByClassName("mydiv")[0].style.borderTop = "4px " + style[1] + color;