JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="input"></textarea>
<textarea id="output"></textarea>

JavaScript

var replacer = (function() {
  var module = {
  'dictionary': {
    'а': '1',
    'б': '2',
    'в': '3',
    'г': '4',
  },
  'unknownSymbol': '?'
};
  function replace(inputString) {
    var outputString = '',
        inputStringArr = inputString.split('');
    for (k in inputStringArr) {
        outputString += (!!module.dictionary[ inputStringArr[k] ]) ? module.dictionary[ inputStringArr[k] ] : module.unknownSymbol;
    }
    return outputString;
  }
  module.run = function(inputElem, outputElem) {
      outputElem.value = replace(inputElem.value);
  };
  return module;
})();

document.getElementById("input").addEventListener("keyup", function() {
  replacer.run(document.getElementById("input"), document.getElementById("output"));
}, false);