JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="input">
<input type="button" id="button2" value="convert" />

JavaScript

$('#button2').click(function() {
  var text = $('#input').val();  
  var result = text.replace(/([A-Z])/g, "_$1");  
  var finalResult = result.toLowerCase();  
  $('#input').val(finalResult);
});