Untitled

<html>
<head>
  <meta charset="utf-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <title>Brian Blessed demo</title>
  <style type="text/css">
    .out {
      font-weight: bold;
    }
    body {
      font: 14/20px sans-serif;
    }
  </style>
</head>
<body>
  Normal: <span class="out normal"></span><br />
  Patched: <span class="out patched"></span>

  <script type="text/javascript">
  // This works with Babel
    class Name {
      constructor(first, last) {
        this.first = first;
        this.last = last;
      }
      getFull() {
        return this.first + ' ' + this.last;
      }
    }

    (function() {
      var orig = Name.prototype.getFull;
      Object.defineProperty(Name.prototype, 'getFull', {
        get: function() {
          if (this.first === 'Brian' && this.last === 'Blessed') {
            return function() {
              return 'I AM BRIAN BLESSED!';
            }
          } else {
            return orig;
          }
        }
      });
    }());

  document.addEventListener('DOMContentLoaded', function() {
    var john = new Name('John', 'Doe');
    var brian = new Name('Brian', 'Blessed');
    document.querySelector('.out.normal').innerHTML = john.getFull();
    // => John Doe
    document.querySelector('.out.patched').innerHTML = brian.getFull();
    // => I AM BRIAN BLESSED!
  });
  </script>
</body>
</html>