JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.0/jquery.min.js"></script>
<h2>Se scrie un sir(cuvant) in ordine inversa</h2>
<input type ='text' id='src'>
<input type = "button" id='apasaBtn' value = "INVERSEAZA SIRUL">
<!-- added by lightpoint -->
<div id='rezultat'>&nbsp</div>

CSS

.stilInit {
						font-family : Courier New;
            font-style: regular;
            /*alte chestii aici daca este nevoie*/
    }
    .stilEroare {
	  				font-size: 32px;
						font-weight: bold;
            color : red;
    }
    
    .stilOk {
	  				font-size: 25px;
						font-weight: bold;
            color : green;
            font-family: Verdana;
            font-style: italic;
    }

JavaScript

//constructor
 var Oglinda =  function(sursaID, destinatieID) {
 	this.sursaID = sursaID;
  this.destinatieID = destinatieID;
 }
 
 //proto
 Oglinda.prototype  = {
		init: function() {
    	$(this.destinatieID).addClass('stilInit');
      return this;
    },
    
    executa: function() {
     	var valoare = this.sursaID.val();
      var lungimeSir =  valoare.length;
		  lungimeSir === 0 ? this.seteazaStilEroare() : this.seteazaStilOk();
      $(this.destinatieID).html(lungimeSir === 0 ?  "EROARE: sirul este vid!" : this.procesare(valoare));
    	return this;
    },
    
    procesare: function(x) {
    	var oglx="";
			for (var i = x.length - 1; i >= 0; i--) {
				oglx = oglx.concat(x[i]);	
			}
		return oglx;
    }, 
    
    seteazaStilEroare: function() {
      if ($(this.destinatieID).hasClass('stilOk')) {
      	$(this.destinatieID).removeClass('stilOk');
      }
      $(this.destinatieID).addClass('stilEroare');
    },
    
    seteazaStilOk: function() {
      if ($(this.destinatieID).hasClass('stilEroare')) {
      	$(this.destinatieID).removeClass('stilEroare');
      }
      $(this.destinatieID).addClass('stilOk');
    }
}


 $(document).ready(function() {
    var oglindaMea = new Oglinda($('#src'), $('#rezultat'));
		$('#apasaBtn').click(function() {
  		oglindaMea.init().executa();
 	 });
 });