third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/test-request-headers-worker.js

// The server injects the request headers here as a JSON string.
const headersAsJson = `%HEADERS%`;
const headers = JSON.parse(headersAsJson);

self.addEventListener('message', async (e) => {
  e.source.postMessage(headers);
});