How to intercept all http requests including form submits

https://developer.mozilla.org/en/docs/Web/API/Service_Worker_API

Service workers essentially act as proxy servers that sit between web applications, and the browser and network (when available).

It takes the form of a JavaScript file that can control the web page/site it is associated with, intercepting and modifying navigation and resource requests

You register a service worker in your application code from a file named, e.g., sw.js by doing:

if ('serviceWorker' in navigator) {
  window.addEventListener('load', function() {
    navigator.serviceWorker.register('sw.js').then(function(registration) {
      console.log('Service worker registered with scope: ', registration.scope);
    }, function(err) {
      console.log('ServiceWorker registration failed: ', err);
    });
  });
}

And in the sw.js file (the actual service-worker code): To intercept requests, you attach a fetch event listener to the service worker that calls the respondWith() method and does something with the .request member from the event object:

self.addEventListener('fetch', function(event) {
  event.respondWith(
    // intercept requests by handling event.request here
  );
});

A simple service worker that just passes through requests unchanged looks like this:

self.addEventListener('fetch', function(event) {
  event.respondWith(
    fetch(event.request)
  );
});

To add a param to the request body, you need to:

1. Serialize the request.
2. Modify that serialized request.
3. Deserialize the modified request to create a new request.
4. Call fetch(…) with that new request.

So, a service worker that does all that would look like this (untested):

self.addEventListener('fetch', function(event) {
  event.respondWith(
    fetchWithParamAddedToRequestBody(event.request)
  );
});
function fetchWithParamAddedToRequestBody(request) {
  serialize(request).then(function(serialized) {
    // modify serialized.body here to add your request parameter
    deserialize(serialized).then(function(request) {
      return fetch(request);
    });
  }); // fixed this
}
function serialize(request) {
  var headers = {};
  for (var entry of request.headers.entries()) {
    headers[entry[0]] = entry[1];
  }
  var serialized = {
    url: request.url,
    headers: headers,
    method: request.method,
    mode: request.mode,
    credentials: request.credentials,
    cache: request.cache,
    redirect: request.redirect,
    referrer: request.referrer
  };  
  if (request.method !== 'GET' && request.method !== 'HEAD') {
    return request.clone().text().then(function(body) {
      serialized.body = body;
      return Promise.resolve(serialized);
    });
  }
  return Promise.resolve(serialized);
}
function deserialize(data) {
  return Promise.resolve(new Request(data.url, data));
}

Note: https://serviceworke.rs/request-deferrer_service-worker_doc.html, a page from the Service Worker Cookbook, is where I lifted that serialize(…) code/approach from—by way of the answer at https://stackoverflow.com/questions/35420980/how-to-alter-the-headers-of-a-request/35421644#35421644—and it’s worth taking a look at, because the code there has detailed annotations explaining what it’s all doing