JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
  <body>
      <div id="charNum">абвгдеёжзийклмнопрстуфхцчшщъыьэюя</div>
      <br>
      <input type="text" id="myTextField" id="field" onkeyup="countChar(this)" size="55"/>
      <br> <br>
      <div id="rep">0</div>
  </body>

JavaScript

var sssr="абвгдеёжзийклмнопрстуфхцчшщъыьэюя";
    var rf = "";
    function countChar(val) {
       rf = sssr;
       var text = val.value; var len = val.value.length;
       for (var i=0; i<len; i++ ) {
       	var s = text.charAt(i);
        rf = rf.replace( s, "");
       }
        $('#charNum').text(rf);
        $('#rep').text(getFrequency(val.value));
      };
      
      
function getFrequency(string) {
var res = "";
  for (var i=0; i<string.length - 1;i++) {
      var char1 = string.charAt(i);
  	  for (var j=i+1;j<string.length;j++) {    
     	  var char2 = string.charAt(j);
        if ( char1 == char2 ) { res = res + char1; }
      }
  }

  return res;
};