JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="3" id="numberField"/>
<span id="result"></span>

JavaScript

var base = 10;
   document.getElementById('numberField').onkeyup = function() {
      if(this.value.length == 0) {
         document.getElementById('result').innerHTML = '';
         return;
      }
      var number = parseInt(this.value);
      if(isNaN(number)) return;
      document.getElementById('result').innerHTML = number * base;
   };
   document.getElementById('numberField').onkeyup();