gtm-hijack-datalayer-push.js

/**
 * THE STANDARD..
 *
 */

(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXXXXX');

/**
 * ..BECOMES --->
 *
 */

window.dataLayer = window.dataLayer || [];
window.dataLayer.pushStashed = window.dataLayer.push;
Object.defineProperty(window.dataLayer, 'push', {
  set(y) {
    this.pushStashed = y;
    Object.defineProperty(this, 'push', {
      writable: true,
      value: function() {
        arguments = [].map.call(arguments, function(event) {
          if (event && event["gtm.element"]) {
            event.dataContext = {}, elem = event["gtm.element"];
            for (; elem && elem !== document; elem = elem.parentNode) {
              [].forEach.call(elem.attributes, function(attr) {
                if (/^data-/.test(attr.name)) {
                  var camelCaseName = attr.name.substr(5).replace(/-(.)/g, function($0, $1) {
                    return $1.toUpperCase();
                  });
                  event.dataContext[camelCaseName] = event.dataContext[camelCaseName] || attr.value;
                }
              });
            }
          }
          return event;
        });
        return this.pushStashed.apply(null,arguments);
      }
    });
  },
  get() {
    return this.pushStashed
  },
  configurable: true
});

(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXXXXX');