Babel

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.6.15/browser.js"></script>
<script type="text/babel">
class Test {
  test() {
    document.write("test");
  }
}

var test = new Test;
test.test(); // "test"
</script>

JavaScript

let x = 5;
alert(x);