How do I replace `serviceWorkerVersion` with `{{flutter_service_worker_version}}` template token?

In my index.html in my flutter web app, I have a serviceWorker script. Flutter now complaints like this:

[   +3 ms] Warning: In index.html:60: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead.

The <body> of the index.html is defined like this:

<body>
  <img src="img/logo_symbol.png" class="center"/>

  <script>
    var serviceWorkerVersion = null;
    var scriptLoaded = false;
    function loadMainDartJs() {
      if (scriptLoaded) {
        return;
      }
      scriptLoaded = true;
      var scriptTag = document.createElement('script');
      scriptTag.src = 'main.dart.js';
      scriptTag.type = 'application/javascript';
      document.body.append(scriptTag);
    }

    if ('serviceWorker' in navigator) {
      // Service workers are supported. Use them.
      window.addEventListener('load', function () {
        // Wait for registration to finish before dropping the <script> tag.
        // Otherwise, the browser will load the script multiple times,
        // potentially different versions.
        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
        navigator.serviceWorker.register(serviceWorkerUrl)
          .then((reg) => {
            function waitForActivation(serviceWorker) {
              serviceWorker.addEventListener('statechange', () => {
                if (serviceWorker.state == 'activated') {
                  console.log('Installed new service worker.');
                  loadMainDartJs();
                }
              });
            }
            if (!reg.active && (reg.installing || reg.waiting)) {
              // No active web worker and we have installed or are installing
              // one for the first time. Simply wait for it to activate.
              waitForActivation(reg.installing || reg.waiting);
            } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
              // When the app updates the serviceWorkerVersion changes, so we
              // need to ask the service worker to update.
              console.log('New service worker available.');
              reg.update();
              waitForActivation(reg.installing);
            } else {
              // Existing service worker is still good.
              console.log('Loading app from service worker.');
              loadMainDartJs();
            }
          });

        // If service worker doesn't succeed in a reasonable amount of time,
        // fallback to plaint <script> tag.
        setTimeout(() => {
          if (!scriptLoaded) {
            console.warn(
              'Failed to load app from service worker. Falling back to plain <script> tag.',
            );
            loadMainDartJs();
          }
        }, 4000);
      });
    } else {
      // Service workers not supported. Just drop the <script> tag.
      loadMainDartJs();
    }
  </script>
</body>

How do I replace serviceWorkerVersion with a {{flutter_service_worker_version}} template instead, as the warning suggests ?

Solution

index.html content might change on Flutter updates, so the best thing I did was creating a brand new flutter project and just copy the generated index.html to my project and voilá. All working fine again.

This is what I got with Flutter version 3.22.0 I added a couple of scripts I needed though.

<!DOCTYPE html>
<html>

    <head>
        <base href="$FLUTTER_BASE_HREF">

        <meta charset="UTF-8">
        <meta content="IE=Edge" http-equiv="X-UA-Compatible">
        <meta name="description" content="A new Flutter project.">

        <!-- iOS meta tags & icons -->
        <meta name="apple-mobile-web-app-capable" content="yes">
        <meta name="apple-mobile-web-app-status-bar-style" content="black">
        <meta name="apple-mobile-web-app-title" content="pockit">
        <link rel="apple-touch-icon" href="icons/Icon-192.png">

        <!-- Favicon -->
        <link rel="icon" type="image/png" href="favicon.png" />

        <title>My project</title>
        <link rel="manifest" href="manifest.json">

        <script src="./assets/packages/mixpanel_flutter/assets/mixpanel.js"></script>
        <script type="application/javascript" src="/assets/packages/flutter_inappwebview_web/assets/web/web_support.js"
                defer></script>
    </head>

    <body>
        <script src="flutter_bootstrap.js" async></script>
    </body>

</html>