Function start

packages/dev/storybook-builder-parcel/preset.mjs:29–82

({options, router})

let watcherSubscription;

export async function start({options, router}) {
  const parcel = await createParcel(options, true);

  // Storybook's CLI runs an Express-style server on :9003 (the user-facing
  // dev URL). Parcel's own dev server runs on :3000 to serve iframe.html and
  // the bundle assets. We mount a reverse proxy on Storybook's router so that
  // any request that isn't the manager's own /index.html falls through to
  // Parcel. If Parcel returns 404 or HTML for a non-iframe URL we hand the
  // request back to next() so Storybook's manager middleware can handle it.
  router.use(async (req, res, next) => {
    if (req.url === '/' || req.url === '/index.html') return next();

    const proxy = createProxyMiddleware({
      target: 'http://localhost:3000/',
      selfHandleResponse: true,
      logLevel: 'warn',
      onProxyRes(proxyRes, req, res) {
        if (
          proxyRes.statusCode === 404 ||
          (proxyRes.headers['content-type']?.startsWith('text/html') &&
            !req.url.startsWith('/iframe.html'))
        ) {
          return next();
        }
        res.statusCode = proxyRes.statusCode;
        for (const header in proxyRes.headers) {
          res.setHeader(header, proxyRes.headers[header]);
        }
        proxyRes.pipe(res);
      }
    });

    const {socket, connection} = req;
    req.socket = null;
    req.connection = null;
    await proxy(req, res, next);
    req.socket = socket;
    req.connection = connection;
  });

  watcherSubscription = await parcel.watch();
  process.on('SIGINT', async () => {
    await watcherSubscription?.unsubscribe();
    process.exit();
  });

  return {
    async bail() {
      await watcherSubscription?.unsubscribe();
    },
    stats: {},
    totalTime: 0
  };
}

export async function build({options}) {
  const parcel = await createParcel(options);