JSFiddle - React, Tailwind, and code Playground

HTML

<html>
    <head>
        <script type="text/javascript" src="http://code.jquery.com/jquery-1.8.2.min.js"></script>
            
    </head>
    <body>
        <form action="" method="post" id="searcher" name="InsertForm" enctype="multipart/form-data">
        
        <input type="text" name="kw">kW
        
        <input type="text" name="ps">PS
    
        </form>
    </body>
</html>

JavaScript

$(document).ready(function() {
    
    function overeni(hodnota) {
        if(isNaN(hodnota)) {
          alert("Zadej číslo!");
          $("input[name='ps']").val(0); 
          $("input[name='kw']").val(0);                  
          return false;           
        } else { return true; }
    }
    
    
    $("input[name='kw']").keyup(function() {
        if(overeni($(this).val())) {
            var kw = Math.round(parseInt($(this).val()) * 0.746);
            $("input[name='ps']").val(kw);
        }
    });
    
     $("input[name='ps']").keyup(function() {
         if(overeni($(this).val())) {
            var kw = Math.round(parseInt($(this).val()) / 0.746);
            $("input[name='kw']").val(kw);
         }
    });
    
});