How to use the altair-static.renderAltair function in altair-static

fs.readFile(filePath, 'utf8', function(err, data) {
        if (err) {
          return console.log('Error loading file to buffer.', err);
        }
        if (filePath && filePath.includes('index.html')) {
          data = renderAltair();
        }

        // Load the data from the file into a buffer and pass it to the callback
        // Using the mime package to get the mime type for the file, based on the file name
        callback({ mimeType: mime.lookup(filePath), data: Buffer.from(data) });
      });
    });

app.get('/', (req, res) => {
    if (req.originalUrl.substr(-1) !== '/') {
      // We need the trailing slash to enable relative file paths to work
      return res.redirect(301, req.originalUrl + '/');
    }
    return res.send(renderAltair(opts));
  });
  app.get('/initial_options.js', (req, res) => {

router.get(url, async (ctx, next) => {

    ctx.body = renderAltair({ baseURL: ctx.url.replace(/\/?$/, '/'), ...opts });

    await next();
  })