server/middleware/load-assets.js
function loadAssets(req, res, next) {
  var {zlSite, socketQuery, appStyles} = res.locals;
  const {config, reCAPTCHAkey} = req.app.locals;

  // load Google analytics
  if (zlSite.zsOptions && zlSite.zsOptions.gtags) {
    let tags = '';
    let gOptimzeSnippetLoaded = false;
    _.forEach(zlSite.zsOptions.gtags, (val, id) => {
      let conf = Object.assign({'send_page_view': false}, val);
      // check if should load Google Optimize page-hiding snippet
      if (!gOptimzeSnippetLoaded && id.startsWith('UA-') && val.optimize_id) {
        let code = fs.readFileSync(`${__dirname}/assets/g-optimze-snippet.js`);
        code = String(code).replace('GTM-XXXXXX', val.optimize_id);
        res.locals.app_headcode.gOptimzeSnippet = {code: code};
        // also need this stylesheet
        appStyles.gOptimzeSnippet = '.async-hide { opacity: 0 !important} ';
        gOptimzeSnippetLoaded = true;

        // we also need to have this function on page
        code = fs.readFileSync(`${__dirname}/assets/set-exp-variant.js`);
        res.locals.app_headcode.setExpVariant = {code: code};
      }
      tags += `gtag('config', '${id}', ${JSON.stringify(conf)});`;
    });
    res.locals.app_headcode.gaSrc = `https://www.googletagmanager.com/gtag/js?` +
      `id=${Object.keys(zlSite.zsOptions.gtags)[0]}`;
    res.locals.app_headcode.gaTracking = {code: `window.dataLayer = ` +
      `window.dataLayer || []; function gtag(){dataLayer.push(arguments);}` +
      `gtag('js', new Date());${tags}`};
  }

  // load socket.io client side (use websocket to support multi-node/clusters)
  res.locals.app_scripts.socket = '/socket.io/socket.io.js';
  res.locals.app_scripts.socketCode = {
    code: `var socket = io.connect('${config.socketUrl}', {query:` +
          `${JSON.stringify(socketQuery)}, 'transports': ['websocket', 'polling']});`};

  // load reCAPTCHA
  res.locals.app_scripts.recaptcha = `https://www.google.com/recaptcha/api.js?render=${reCAPTCHAkey}`;
  res.locals.app_scripts.recaptchaCode = {
    code: `var reCAPTCHAkey='${reCAPTCHAkey}'; grecaptcha.ready(function() { `+
          `grecaptcha.execute('${reCAPTCHAkey}', {action: 'landing_page'})});`
  }

  // load other client provided custom codes
  if (zlSite.zsOptions && zlSite.zsOptions.customCode) {
    _.forEach(zlSite.zsOptions.customCode, (val, key) => {
      let js = customCode.render(val.code, key);
      if (!_.isEmpty(js)) {
        switch (val.location) {
          case 'body-start':
            res.locals.app_bodycode = Object.assign(res.locals.app_bodycode, js);
            break;
          case 'body-end':
            res.locals.app_scripts = Object.assign(res.locals.app_scripts, js);
            break;
          case 'head':
            res.locals.app_headcode = Object.assign(res.locals.app_headcode, js);
        }
      }
    });
  }

  // load js
  if ('development' === req.app.get('env') && process.env.DONT_BUNDLE) {
    // when development with DONT_BUNDLE
    var wpManifest = {
      'runtime.js': `${zlSite.zsId}/dist/js/runtime.js`,
      'vendor.js': `${zlSite.zsId}/dist/js/vendor.js`,
      'app.js': `${zlSite.zsId}/dist/js/app.js`,
      'app.css': `${zlSite.zsId}/dist/css/app.css`,
    };
    var port = 3000;
    if (config.webpackDevServer && config.webpackDevServer.port) {
      port = config.webpackDevServer.port;
    }
    var assetPath = `//${req.hostname}:${port}/`;
  }
  else {
    // when in production
    var wpManifest = require(`${process.cwd()}/${zlSite.contentPath}/dist/manifest.json`);
    var assetPath = '/';
    if (zlSite.zsBaseUrlPath) {
      assetPath += `${zlSite.zsBaseUrlPath}/`;
    }

    // if not verified that the files exist
    if (!app.locals.bundleVerifiedExist[zlSite.zsId]) {
      // verify
      _.forEach(wpManifest, (val) => {
        // ignore fonts
        if (val.includes('./fonts/')) return;

        let url = `${req.protocol}://${req.hostname}${assetPath}${val}`;
        axios.get(url)
          .then(response => req.app.logger.info('bundle file verified:', url))
          .catch(error => {
            // looks like we couldn't find it
            req.app.error('bundle file missing: %s %o', url, error)
          });
      });

      // set in state so we don't check again
      app.locals.bundleVerifiedExist[zlSite.zsId] = 1;
    }
  }

  if (wpManifest) {
    if (wpManifest['runtime.js']) {
      res.locals.app_scripts.runtime = `${assetPath}${wpManifest['runtime.js']}`;
    }
    if (wpManifest['vendor.js']) {
      res.locals.app_scripts.vendor = `${assetPath}${wpManifest['vendor.js']}`;
    }
    if (wpManifest['app.js']) {
      res.locals.app_scripts.app = `${assetPath}${wpManifest['app.js']}`;
    }
    if (wpManifest['app.css']) {
      res.locals.app_stylesheets.app = `${assetPath}${wpManifest['app.css']}`;
    }
  }

  next();
}
function loadAssets(req, res, next) {
  var {zlSite, socketQuery, appStyles} = res.locals;
  const {config, reCAPTCHAkey} = req.app.locals;

  // load Google analytics
  if (zlSite.zsOptions && zlSite.zsOptions.gtags) {
    let tags = '';
    let gOptimzeSnippetLoaded = false;
    _.forEach(zlSite.zsOptions.gtags, (val, id) => {
      let conf = Object.assign({'send_page_view': false}, val);
      // check if should load Google Optimize page-hiding snippet
      if (!gOptimzeSnippetLoaded && id.startsWith('UA-') && val.optimize_id) {
        let code = fs.readFileSync(`${__dirname}/assets/g-optimze-snippet.js`);
        code = String(code).replace('GTM-XXXXXX', val.optimize_id);
        res.locals.app_headcode.gOptimzeSnippet = {code: code};
        // also need this stylesheet
        appStyles.gOptimzeSnippet = '.async-hide { opacity: 0 !important} ';
        gOptimzeSnippetLoaded = true;

        // we also need to have this function on page
        code = fs.readFileSync(`${__dirname}/assets/set-exp-variant.js`);
        res.locals.app_headcode.setExpVariant = {code: code};
      }
      tags += `gtag('config', '${id}', ${JSON.stringify(conf)});`;
    });
    res.locals.app_headcode.gaSrc = `https://www.googletagmanager.com/gtag/js?` +
      `id=${Object.keys(zlSite.zsOptions.gtags)[0]}`;
    res.locals.app_headcode.gaTracking = {code: `window.dataLayer = ` +
      `window.dataLayer || []; function gtag(){dataLayer.push(arguments);}` +
      `gtag('js', new Date());${tags}`};
  }

  // load socket.io client side (use websocket to support multi-node/clusters)
  res.locals.app_scripts.socket = '/socket.io/socket.io.js';
  res.locals.app_scripts.socketCode = {
    code: `var socket = io.connect('${config.socketUrl}', {query:` +
          `${JSON.stringify(socketQuery)}, 'transports': ['websocket', 'polling']});`};

  // load reCAPTCHA
  res.locals.app_scripts.recaptcha = `https://www.google.com/recaptcha/api.js?render=${reCAPTCHAkey}`;
  res.locals.app_scripts.recaptchaCode = {
    code: `var reCAPTCHAkey='${reCAPTCHAkey}'; grecaptcha.ready(function() { `+
          `grecaptcha.execute('${reCAPTCHAkey}', {action: 'landing_page'})});`
  }

  // load other client provided custom codes
  if (zlSite.zsOptions && zlSite.zsOptions.customCode) {
    _.forEach(zlSite.zsOptions.customCode, (val, key) => {
      let js = customCode.render(val.code, key);
      if (!_.isEmpty(js)) {
        switch (val.location) {
          case 'body-start':
            res.locals.app_bodycode = Object.assign(res.locals.app_bodycode, js);
            break;
          case 'body-end':
            res.locals.app_scripts = Object.assign(res.locals.app_scripts, js);
            break;
          case 'head':
            res.locals.app_headcode = Object.assign(res.locals.app_headcode, js);
        }
      }
    });
  }

  // load js
  if ('development' === req.app.get('env') && process.env.DONT_BUNDLE) {
    // when development with DONT_BUNDLE
    var wpManifest = {
      'runtime.js': `${zlSite.zsId}/dist/js/runtime.js`,
      'vendor.js': `${zlSite.zsId}/dist/js/vendor.js`,
      'app.js': `${zlSite.zsId}/dist/js/app.js`,
      'app.css': `${zlSite.zsId}/dist/css/app.css`,
    };
    var port = 3000;
    if (config.webpackDevServer && config.webpackDevServer.port) {
      port = config.webpackDevServer.port;
    }
    var assetPath = `//${req.hostname}:${port}/`;
  }
  else {
    // when in production
    var wpManifest = require(`${process.cwd()}/${zlSite.contentPath}/dist/manifest.json`);
    var assetPath = '/';
    if (zlSite.zsBaseUrlPath) {
      assetPath += `${zlSite.zsBaseUrlPath}/`;
    }

    // if not verified that the files exist
    if (!app.locals.bundleVerifiedExist[zlSite.zsId]) {
      // verify
      _.forEach(wpManifest, (val) => {
        // ignore fonts
        if (val.includes('./fonts/')) return;

        let url = `${req.protocol}://${req.hostname}${assetPath}${val}`;
        axios.get(url)
          .then(response => req.app.logger.info('bundle file verified:', url))
          .catch(error => {
            // looks like we couldn't find it
            req.app.error('bundle file missing: %s %o', url, error)
          });
      });

      // set in state so we don't check again
      app.locals.bundleVerifiedExist[zlSite.zsId] = 1;
    }
  }

  if (wpManifest) {
    if (wpManifest['runtime.js']) {
      res.locals.app_scripts.runtime = `${assetPath}${wpManifest['runtime.js']}`;
    }
    if (wpManifest['vendor.js']) {
      res.locals.app_scripts.vendor = `${assetPath}${wpManifest['vendor.js']}`;
    }
    if (wpManifest['app.js']) {
      res.locals.app_scripts.app = `${assetPath}${wpManifest['app.js']}`;
    }
    if (wpManifest['app.css']) {
      res.locals.app_stylesheets.app = `${assetPath}${wpManifest['app.css']}`;
    }
  }

  next();
}
loads assets