CORS Test (Chrome, Firefox) - This extension will begin performing HTTP requests against a bespoke test server immediately after installation. To view the results, open a devtools session for the extension's background context and look through the console output.

background.js

globalThis.browser ??= globalThis.chrome;

let testCounter = 0;
let corsTest = async (
  {
    client: {
      forcePreflight = false,
      mode = undefined,
    },
    server: {
      cors = false,
    },
  } = {}
) => {
  const options = {};
  let url = new URL("https://dotproto-cors-test.glitch.me/script.js");
  // Include timestamp to prevent cache hits
  url.searchParams.set('time', Date.now());

  let corsValue = 'false';
  let corsColor = 'slategray';
  if (cors) {
    url.searchParams.set('cors', true);
    corsValue = 'true';
    corsColor = 'deepskyblue';
  }

  let modeValue = 'none';
  if (mode) {
    options.mode = mode;
    modeValue = mode;
  }
  let modeColor = 'slategray';
  switch (mode) {
    case 'no-cors':
      modeColor = 'orange';
      break;
    case 'cors':
      modeColor = 'salmon';
      break;
  }

  let preflightValue = false;
  let preflightColor = 'slategray';
  if (forcePreflight) {
    options.headers = { 'unsafe-header': true };
    preflightValue = true;
    preflightColor = 'gold';
  }

  console.groupCollapsed(`Test ${(++testCounter).toString().padStart(2, '0')} - ${url} ${JSON.stringify(options)}`);
  console.log(`Request  --> CORS mode:  %c${modeValue.padStart(7, ' ')}%c | force preflight: %c${preflightValue}`, `color: ${modeColor};`, `color: initial;`, `color: ${preflightColor}`);
  console.log(`Response <-- CORS headers: %c${corsValue.padStart(5, ' ')}`, `color: ${corsColor}`);
  console.log(`URL: ${url}`);
  console.groupEnd();

  let resultValue = 'SUCCESS';
  let resultColor = 'aquamarine';
  try {
    await fetch(url, options);
  } catch {
    resultValue = 'ERROR';
    resultColor = 'red';
  }

  console.log(`%c${resultValue}`, `color: ${resultColor}`);

  await sleep();
}

browser.runtime.onInstalled.addListener(async () => {
  let result;

  // Set 1

  await corsTest({
    client: { mode: false, forcePreflight: false },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: false, forcePreflight: true },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: false, forcePreflight: false },
    server: { cors: true },
  });

  await corsTest({
    client: { mode: false, forcePreflight: true },
    server: { cors: true },
  });

  // Set 2

  await corsTest({
    client: { mode: 'no-cors', forcePreflight: false },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: 'no-cors', forcePreflight: true },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: 'no-cors', forcePreflight: false },
    server: { cors: true },
  });

  await corsTest({
    client: { mode: 'no-cors', forcePreflight: true },
    server: { cors: true },
  });

  // Set 3

  await corsTest({
    client: { mode: 'cors', forcePreflight: false },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: 'cors', forcePreflight: true },
    server: { cors: false },
  });

  await corsTest({
    client: { mode: 'cors', forcePreflight: false },
    server: { cors: true },
  });

  await corsTest({
    client: { mode: 'cors', forcePreflight: true },
    server: { cors: true },
  });

});

async function sleep(time = 1000) {
  return new Promise(resolve => {
    setTimeout(() => resolve(), time);
  });
}

manifest.json

{
  "name": "CORS test",
  "version": "1.0",
  "manifest_version": 3,
  "background": {
    "scripts": ["background.js"],
    "service_worker": "background.js"
  },
  "action": {},
  "permissions": ["*://*/*"],
  "host_permissions": ["*://*/*"]
}