postcode.html.erb

<%= react_component('PostcodeCheck') %>

postcode.js.jsx

var PostcodeCheck = React.createClass({
  render: function() {
    return (
      <section>
        <form id="postcodecheck">
          <input id="postcode" name="postcode" placeholder="1234AA">
          <input id="housenumber" name="housenumber" placeholder="1">
          <input id="housenumber_addition" name="housenumber_addition" placeholder="a">
          <input type="submit" value="Check!" class="next-button">
          <p class="postcodecheck-result"></p>
        </form>
      </section>
    );
  }
});