content/static/html/pages/notfound.tmpl

<!--
	Copyright 2019 The Go Authors. All rights reserved.
	Use of this source code is governed by a BSD-style
	license that can be found in the LICENSE file.
-->

{{define "main_content"}}
<div class="Container">
  <div class="Content">
    <img class="NotFound-gopher" src="/static/img/gopher-airplane.svg" alt="The Go Gopher">
    {{template "message" .MessageData}}
    <div class="NotFound-container">
      <button class="NotFound-button js-notFoundButton">Fetch</button>
    </div>
  </div>
</div>

<!--  TODO: update middleware.AcceptMethods so that the GET at the end of the
      script can be a POST instead.
     Do not add comments to the script below: they are stripped by html/template
     (https://golang.org/issue/28628), which messes up the CSP hash.
-->     
<script>
const fetchButton = document.querySelector('.js-notFoundButton');
if (fetchButton) {
  fetchButton.addEventListener('click', e => {
    e.preventDefault();
    fetchPath()
  });
}
function fetchPath() {
  httpRequest = new XMLHttpRequest();
  var btn = document.querySelector('.js-notFoundButton');
  btn.disabled = true;
  btn.className = 'NotFound-button-disabled';

  if (!httpRequest) {
    alert('Giving up :( Cannot create an XMLHTTP instance');
    return false;
  }
  httpRequest.onreadystatechange = function(){
    if (httpRequest.readyState === XMLHttpRequest.DONE) {
      if (httpRequest.status === 200) {
      	location.reload();
      } else {
         document.querySelector('.js-notFoundMessage').innerHTML = httpRequest.responseText;
         btn.innerHTML = 'Failed';
      }
    }
  };
  document.querySelector('.js-notFoundMessage').innerHTML = "Fetching... Feel free to navigate away and check back later, we'll keep working on it!";
  btn.innerHTML = "Fetching...";
  httpRequest.open('GET', "/fetch" + window.location.pathname);
  httpRequest.send();
}
</script>
{{end}}