JSFiddle - React, Tailwind, and code Playground

HTML

Width <input type="Text" name="width" id="width" size="4">  
Height <input type="Text" id="height" name="height" size="4" onkeyup=""> <br><br>
Area <input type="Text" name="area" id="area" size="4"> <br><br>
<input type="button" name="calculate" id="area" size="4" onclick="calculateArea()" value="calculate">

JavaScript

function calculateArea() {
  var width = +document.getElementById("width").value;
  var height =+document.getElementById("height").value;
  var area = width * height;
  document.getElementById("area").value = area;

}