JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
  <head>
    <link href="http://netdna.bootstrapcdn.com/twitter-bootstrap/2.2.1/css/bootstrap-combined.min.css" rel="stylesheet">
    <meta charset="utf-8">
  </head>
  <body>
    <div class="container-fluid">
      <div class="page-header">
        <h1>
          Codigo Verificador Patente
        </h1>
      </div>
      <div class="row">
        <div class="span4">
          <form class="well">
            <fieldset>
              <legend>Ingresa los Datos</legend>
              <label>Patente</label>
              <input type="text" placeholder="USW 621" id="plate" class="input-small">
            </fieldset>
            <button type="submit" class="btn btn-primary">Procesar</button>
          </form>
        </div>
        <div class="span8">
          <pre id="resultados">
            Sin Resultados...
          </pre>
        </div>
      </div>
        <h3>
            Utilizar aqui: <a target="_blank" href="https://lbserver02.agip.gob.ar/ConsultaPat/">https://lbserver02.agip.gob.ar/ConsultaPat/</a> (puede variar)
        </h3>        
  </body>
</html>

JavaScript

var plate_ = "USW 621";

var letters = {
   "A": "14",
   "B": "01",
   "C": "0",
   "D": "16",
   "E": "05",
   "F": "20",
   "G": "19",
   "H": "09",
   "I": "24",
   "J": "07",
   "K": "21",
   "L": "08",
   "M": "04",
   "N": "13",
   "O": "25",
   "P": "22",
   "Q": "18",
   "R": "10",
   "S": "02",
   "T": "06",
   "U": "12",
   "V": "23",
   "W": "11",
   "X": "03",
   "Y": "15",
   "Z": "17",
   "1": "1",
   "2": "2",
   "3": "3",
   "4": "4",
   "5": "5",
   "6": "6",
   "7": "7",
   "8": "8",
   "9": "9"
}

function parsePlate(plate){
  plate = plate.replace(' ','');
  var crypted = "",
      even = 0,
      odd = 0;
  
  for (var idx = 0; idx < plate.length; idx++) {
    crypted += letters[plate[idx]];
  }

  for (var idx = crypted.length - 1; idx >= 0; idx--) {
    if(idx % 2 == 1) {
      odd += crypted[idx]*1;
    } else {
      even += crypted[idx]*1;
    }
  }

  if (even > 10) even = (even+"")[0]*1+(even+"")[1]*1;
  if (odd > 10) odd = (odd+"")[0]*1+(odd+"")[1]*1;
  
  return even+""+odd;
}
   
document.querySelector("form").addEventListener("submit", function(e){
  e.preventDefault();
  document.querySelector("#resultados").innerHTML = parsePlate(document.querySelector('#plate').value);
})