gecko-dev/dom/serviceworkers/test/fetch.js

function get_query_params(url) {
  var search = new URL(url).search;
  if (!search) {
    return {};
  }
  var ret = {};
  var params = search.substring(1).split("&");
  params.forEach(function(param) {
    var element = param.split("=");
    ret[decodeURIComponent(element[0])] = decodeURIComponent(element[1]);
  });
  return ret;
}

addEventListener("fetch", function(event) {
  if (event.request.url.includes("fail.html")) {
    event.respondWith(fetch("hello.html", { integrity: "abc" }));
  } else if (event.request.url.includes("fake.html")) {
    event.respondWith(fetch("hello.html"));
  } else if (event.request.url.includes("file_js_cache")) {
    event.respondWith(fetch(event.request));
  } else if (event.request.url.includes("redirect")) {
    let param = get_query_params(event.request.url);
    let url = param.url;
    let mode = param.mode;

    event.respondWith(fetch(url, { mode }));
  }
});

addEventListener("activate", function(event) {
  event.waitUntil(clients.claim());
});