JSFiddle - React, Tailwind, and code Playground

HTML

<div id="outer">
  <div id="foo"></div>
  <center>Days</center>
 </div>

CSS

html{
  font-family: Sans-Serif;
}

#outer
{
  color: white; 
  text-shadow: 0px 0px 10px rgba(0, 0, 0, .5);
  font-weight: 100;
  width: 50px;
  height: 70px;
  background: #1BBC9B;  
  border-radius: 3px;
  padding: 10px;
  line-height: 30px;
}

#foo {
  color: white;
  text-shadow: 0px 0px 10px rgba(0, 0, 0, .5);
  width: 50px;
  height: 50px;
  background: #049372;
  border-radius: 3px;
  font-size: 24px;
  text-align: center;
  line-height: 50px;
}

JavaScript

var value = 10 // Variable definieren und Wert zuweisen
document.getElementById("foo").append(value) //div mit der id 'foo' um Variable 'value' erweitern