Intercepting GA4 Events

This guide covers methods for intercepting Google Analytics 4 (GA4) events. Choose the method that best fits your needs.

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Method 1: Duplicate GA4 Requests (Recommended)

This method keeps your existing GA4 setup intact while duplicating events to a custom endpoint. Your data will continue flowing to Google Analytics as normal.

Steps:

1. Open Google Tag Manager

   • Navigate to your GTM container
   • Go to the Tags section

2. Create a Custom HTML Tag

   • Click New to create a new tag
   • Choose Custom HTML as the tag type
   • Name it: GA4 Request Duplicator

3. Add Your Interception Code

   • Paste your custom code into the HTML field (remember to edit the line containing endpoint URL!):

   Note: This code duplicates all Google Analytics 4 (GA4) tracking requests and redirects them to a custom endpoint while keeping the original Google Analytics functionality intact. It intercepts browser API calls (XMLHttpRequest, fetch, navigator.sendBeacon, and script src assignments) that websites commonly use to send tracking data.

Minified (for production)

<script>
var hitbox_endpoint = "https://example.org/g/collect"; // Replace it with your endpoint URL

(function(){function isGA4CollectUrl(url){if(!url||typeof url!=="string")return false;try{var parsed=new URL(url,location.href);var duplicateTarget=new URL(hitbox_endpoint,location.href);if(parsed.origin===duplicateTarget.origin&&parsed.pathname===duplicateTarget.pathname){return false}var params=parsed.searchParams;var hasGtm=params.has("gtm");var hasTagExp=params.has("tag_exp");var measurementId=params.get("tid")||params.get("id")||"";var isMeasurementIdGA4=/^G-[A-Z0-9]+$/i.test(measurementId);return hasGtm&&hasTagExp&&isMeasurementIdGA4}catch(e){if(typeof url==="string"){if(url.indexOf(hitbox_endpoint)!==-1)return false;var hasGtmFallback=url.indexOf("gtm=")!==-1;var hasTagExpFallback=url.indexOf("tag_exp=")!==-1;var idMatch=url.match(/[?&](?:tid|id)=G-[A-Za-z0-9]+/);return !!(hasGtmFallback&&hasTagExpFallback&&idMatch)}return false}}var originalXHROpen=XMLHttpRequest.prototype.open;var originalXHRSend=XMLHttpRequest.prototype.send;XMLHttpRequest.prototype.open=function(method,url){this._requestMethod=method;this._requestUrl=url;return originalXHROpen.apply(this,arguments)};XMLHttpRequest.prototype.send=function(body){if(this._requestUrl&&isGA4CollectUrl(this._requestUrl)){var originalResult=originalXHRSend.apply(this,arguments);try{var method=(this._requestMethod||"GET").toUpperCase();if(method==="GET"){var parsedUrl=new URL(this._requestUrl,location.href);var dupUrl=new URL(hitbox_endpoint,location.href);dupUrl.search=parsedUrl.search;fetch(dupUrl.toString(),{method:"GET",keepalive:true}).catch(function(error){console.error("gtm /g/collect interceptor: error duplicating GET xhr:",error)})}else if(method==="POST"){var parsedPostUrl=new URL(this._requestUrl,location.href);var dupPostUrl=new URL(hitbox_endpoint,location.href);dupPostUrl.search=parsedPostUrl.search;fetch(dupPostUrl.toString(),{method:"POST",body:body,keepalive:true}).catch(function(error){console.error("gtm /g/collect interceptor: error duplicating POST xhr:",error)})}}catch(dupErr){console.error("gtm /g/collect interceptor: xhr duplication failed:",dupErr)}return originalResult}return originalXHRSend.apply(this,arguments)};var originalFetch=window.fetch;window.fetch=function(resource,config){var requestUrl=typeof resource==="string"?resource:resource&&resource.url;var method=(config&&config.method)||(resource&&resource.method)||"GET";if(isGA4CollectUrl(requestUrl)){var upperMethod=(method||"GET").toUpperCase();var prepareBodyPromise=Promise.resolve(undefined);if(upperMethod==="POST"){if(config&&Object.prototype.hasOwnProperty.call(config,"body")){prepareBodyPromise=Promise.resolve(config.body)}else if(typeof Request!=="undefined"&&resource instanceof Request){try{var clonedReq=resource.clone();prepareBodyPromise=clonedReq.blob().catch(function(){return undefined})}catch(e){prepareBodyPromise=Promise.resolve(undefined)}}}var originalPromise=originalFetch.apply(this,arguments);var duplicateUrl;try{var parsed=new URL(requestUrl,location.href);var dup=new URL(hitbox_endpoint,location.href);dup.search=parsed.search;duplicateUrl=dup.toString()}catch(e){duplicateUrl=hitbox_endpoint}if(upperMethod==="GET"){fetch(duplicateUrl,{method:"GET",keepalive:true}).catch(function(error){console.error("gtm /g/collect interceptor: error duplicating GET fetch:",error)})}else if(upperMethod==="POST"){prepareBodyPromise.then(function(dupBody){fetch(duplicateUrl,{method:"POST",body:dupBody,keepalive:true}).catch(function(error){console.error("gtm /g/collect interceptor: error duplicating POST fetch:",error)})})}return originalPromise}return originalFetch.apply(this,arguments)};if(navigator.sendBeacon){var originalSendBeacon=navigator.sendBeacon;navigator.sendBeacon=function(url,data){if(isGA4CollectUrl(url)){var originalResult=originalSendBeacon.apply(this,arguments);try{var parsedBeaconUrl=new URL(url,location.href);var dupBeaconUrl=new URL(hitbox_endpoint,location.href);dupBeaconUrl.search=parsedBeaconUrl.search;originalSendBeacon.call(navigator,dupBeaconUrl.toString(),data)}catch(e){console.error("gtm /g/collect interceptor: error duplicating sendBeacon:",e)}return originalResult}return originalSendBeacon.apply(this,arguments)}}try{var scriptSrcDescriptor=Object.getOwnPropertyDescriptor(HTMLScriptElement.prototype,"src");var originalScriptSrcSetter=scriptSrcDescriptor&&scriptSrcDescriptor.set;var originalScriptSrcGetter=scriptSrcDescriptor&&scriptSrcDescriptor.get;var originalScriptSetAttribute=HTMLScriptElement.prototype.setAttribute;var duplicateIfGA4Url=function(urlString){try{if(!isGA4CollectUrl(urlString))return;var parsed=new URL(urlString,location.href);var dup=new URL(hitbox_endpoint,location.href);dup.search=parsed.search;fetch(dup.toString(),{method:"GET",keepalive:true}).catch(function(error){console.error("gtm /g/collect interceptor: error duplicating script GET:",error)})}catch(e){}};if(originalScriptSrcSetter&&originalScriptSrcGetter){Object.defineProperty(HTMLScriptElement.prototype,"src",{configurable:true,enumerable:true,get:function(){return originalScriptSrcGetter.call(this)},set:function(value){try{var last=this.__ga4LastSrcDuplicated;if(value&&value!==last){duplicateIfGA4Url(String(value));this.__ga4LastSrcDuplicated=String(value)}var self=this;var onloadOnce=function(){try{var finalUrl=self.src;if(finalUrl&&finalUrl!==self.__ga4LastSrcDuplicated){duplicateIfGA4Url(finalUrl);self.__ga4LastSrcDuplicated=finalUrl}}catch(_){}self.removeEventListener("load",onloadOnce)};this.addEventListener("load",onloadOnce)}catch(_){}return originalScriptSrcSetter.call(this,value)}})}HTMLScriptElement.prototype.setAttribute=function(name,value){try{if(String(name).toLowerCase()==="src"){var v=String(value);var last=this.__ga4LastSrcDuplicated;if(v&&v!==last){duplicateIfGA4Url(v);this.__ga4LastSrcDuplicated=v}var selfAttr=this;var onloadOnceAttr=function(){try{var finalUrlAttr=selfAttr.src;if(finalUrlAttr&&finalUrlAttr!==selfAttr.__ga4LastSrcDuplicated){duplicateIfGA4Url(finalUrlAttr);selfAttr.__ga4LastSrcDuplicated=finalUrlAttr}}catch(_){}selfAttr.removeEventListener("load",onloadOnceAttr)};this.addEventListener("load",onloadOnceAttr)}}catch(_){}return originalScriptSetAttribute.apply(this,arguments)}}catch(e){}})();
</script>

Non-minified (for auditing)

<script>
var hitbox_endpoint = "https://example.org/g/collect"; // Replace it with your endpoint URL

(function () {
  function isGA4CollectUrl(url) {
    if (!url || typeof url !== "string") return false;
    try {
      var parsed = new URL(url, location.href);
      // Recursion guard: do not intercept our own duplicate endpoint
      var duplicateTarget = new URL(hitbox_endpoint, location.href);
      if (
        parsed.origin === duplicateTarget.origin &&
        parsed.pathname === duplicateTarget.pathname
      ) {
        return false;
      }

      var params = parsed.searchParams;
      var hasGtm = params.has("gtm");
      var hasTagExp = params.has("tag_exp");
      var measurementId = params.get("tid") || params.get("id") || "";
      var isMeasurementIdGA4 = /^G-[A-Z0-9]+$/i.test(measurementId);

      return hasGtm && hasTagExp && isMeasurementIdGA4;
    } catch (e) {
      // Fallback for non-standard/relative URLs
      if (typeof url === "string") {
        if (url.indexOf(hitbox_endpoint) !== -1) return false;
        var hasGtmFallback = url.indexOf("gtm=") !== -1;
        var hasTagExpFallback = url.indexOf("tag_exp=") !== -1;
        var idMatch = url.match(/[?&](?:tid|id)=G-[A-Za-z0-9]+/);
        return !!(hasGtmFallback && hasTagExpFallback && idMatch);
      }
      return false;
    }
  }

  // --- monkey-patch for XMLHttpRequest --
  var originalXHROpen = XMLHttpRequest.prototype.open;
  var originalXHRSend = XMLHttpRequest.prototype.send;

  XMLHttpRequest.prototype.open = function (method, url) {
    this._requestMethod = method;
    this._requestUrl = url;
    return originalXHROpen.apply(this, arguments);
  };

  XMLHttpRequest.prototype.send = function (body) {
    if (this._requestUrl && isGA4CollectUrl(this._requestUrl)) {
      // First send original request to Google Analytics
      var originalResult = originalXHRSend.apply(this, arguments);
      // Then send duplicate to our endpoint mimicking method and payload
      try {
        var method = (this._requestMethod || "GET").toUpperCase();
        if (method === "GET") {
          var parsedUrl = new URL(this._requestUrl, location.href);
          var dupUrl = new URL(hitbox_endpoint, location.href);
          dupUrl.search = parsedUrl.search; // copy exact query string
          fetch(dupUrl.toString(), {
            method: "GET",
            keepalive: true,
          }).catch(function (error) {
            console.error(
              "gtm /g/collect interceptor: error duplicating GET xhr:",
              error
            );
          });
        } else if (method === "POST") {
          // Reuse the original body as-is and preserve original query string
          var parsedPostUrl = new URL(this._requestUrl, location.href);
          var dupPostUrl = new URL(hitbox_endpoint, location.href);
          dupPostUrl.search = parsedPostUrl.search;
          fetch(dupPostUrl.toString(), {
            method: "POST",
            body: body,
            keepalive: true,
          }).catch(function (error) {
            console.error(
              "gtm /g/collect interceptor: error duplicating POST xhr:",
              error
            );
          });
        }
      } catch (dupErr) {
        console.error(
          "gtm /g/collect interceptor: xhr duplication failed:",
          dupErr
        );
      }
      return originalResult;
    }
    return originalXHRSend.apply(this, arguments);
  };

  // --- monkey-patch for fetch ---
  var originalFetch = window.fetch;
  window.fetch = function (resource, config) {
    var requestUrl =
      typeof resource === "string" ? resource : resource && resource.url;
    var method =
      (config && config.method) || (resource && resource.method) || "GET";

    if (isGA4CollectUrl(requestUrl)) {
      // Prepare duplicate details
      var upperMethod = (method || "GET").toUpperCase();

      // If we need the body from a Request, clone BEFORE calling the original fetch
      var prepareBodyPromise = Promise.resolve(undefined);
      if (upperMethod === "POST") {
        if (config && Object.prototype.hasOwnProperty.call(config, "body")) {
          prepareBodyPromise = Promise.resolve(config.body);
        } else if (
          typeof Request !== "undefined" &&
          resource instanceof Request
        ) {
          try {
            var clonedReq = resource.clone();
            prepareBodyPromise = clonedReq.blob().catch(function () {
              return undefined;
            });
          } catch (e) {
            prepareBodyPromise = Promise.resolve(undefined);
          }
        }
      }

      // First send original request to Google Analytics
      var originalPromise = originalFetch.apply(this, arguments);

      // Compute duplicate URL (copy exact query string)
      var duplicateUrl;
      try {
        var parsed = new URL(requestUrl, location.href);
        var dup = new URL(hitbox_endpoint, location.href);
        dup.search = parsed.search;
        duplicateUrl = dup.toString();
      } catch (e) {
        duplicateUrl = hitbox_endpoint;
      }

      // Then send duplicate to our endpoint mimicking method and payload
      if (upperMethod === "GET") {
        fetch(duplicateUrl, { method: "GET", keepalive: true }).catch(function (
          error
        ) {
          console.error(
            "gtm /g/collect interceptor: error duplicating GET fetch:",
            error
          );
        });
      } else if (upperMethod === "POST") {
        prepareBodyPromise.then(function (dupBody) {
          // If we cloned a Request and got a Blob, use it; otherwise use provided body
          fetch(duplicateUrl, {
            method: "POST",
            body: dupBody,
            keepalive: true,
          }).catch(function (error) {
            console.error(
              "gtm /g/collect interceptor: error duplicating POST fetch:",
              error
            );
          });
        });
      }
      return originalPromise;
    }
    return originalFetch.apply(this, arguments);
  };

  // --- monkey-patch for navigator.sendBeacon ---
  if (navigator.sendBeacon) {
    var originalSendBeacon = navigator.sendBeacon;
    navigator.sendBeacon = function (url, data) {
      if (isGA4CollectUrl(url)) {
        // First send original request to Google Analytics
        var originalResult = originalSendBeacon.apply(this, arguments);
        // Then send duplicate to our endpoint with the same body and query string
        try {
          var parsedBeaconUrl = new URL(url, location.href);
          var dupBeaconUrl = new URL(hitbox_endpoint, location.href);
          dupBeaconUrl.search = parsedBeaconUrl.search;
          originalSendBeacon.call(navigator, dupBeaconUrl.toString(), data);
        } catch (e) {
          console.error(
            "gtm /g/collect interceptor: error duplicating sendBeacon:",
            e
          );
        }
        return originalResult;
      }
      return originalSendBeacon.apply(this, arguments);
    };
  }

  // --- monkey-patch for <script src=...> loads ---
  try {
    var scriptSrcDescriptor = Object.getOwnPropertyDescriptor(
      HTMLScriptElement.prototype,
      "src"
    );
    var originalScriptSrcSetter =
      scriptSrcDescriptor && scriptSrcDescriptor.set;
    var originalScriptSrcGetter =
      scriptSrcDescriptor && scriptSrcDescriptor.get;
    var originalScriptSetAttribute = HTMLScriptElement.prototype.setAttribute;

    var duplicateIfGA4Url = function (urlString) {
      try {
        if (!isGA4CollectUrl(urlString)) return;
        var parsed = new URL(urlString, location.href);
        var dup = new URL(hitbox_endpoint, location.href);
        dup.search = parsed.search;
        fetch(dup.toString(), { method: "GET", keepalive: true }).catch(
          function (error) {
            console.error(
              "gtm /g/collect interceptor: error duplicating script GET:",
              error
            );
          }
        );
      } catch (e) {
        // swallow
      }
    };

    if (originalScriptSrcSetter && originalScriptSrcGetter) {
      Object.defineProperty(HTMLScriptElement.prototype, "src", {
        configurable: true,
        enumerable: true,
        get: function () {
          return originalScriptSrcGetter.call(this);
        },
        set: function (value) {
          try {
            var last = this.__ga4LastSrcDuplicated;
            if (value && value !== last) {
              duplicateIfGA4Url(String(value));
              this.__ga4LastSrcDuplicated = String(value);
            }
            // Also duplicate after load to capture redirected/final URL with query params
            var self = this;
            var onloadOnce = function () {
              try {
                var finalUrl = self.src;
                if (finalUrl && finalUrl !== self.__ga4LastSrcDuplicated) {
                  duplicateIfGA4Url(finalUrl);
                  self.__ga4LastSrcDuplicated = finalUrl;
                }
              } catch (_) {}
              self.removeEventListener("load", onloadOnce);
            };
            this.addEventListener("load", onloadOnce);
          } catch (_) {}
          return originalScriptSrcSetter.call(this, value);
        },
      });
    }

    HTMLScriptElement.prototype.setAttribute = function (name, value) {
      try {
        if (String(name).toLowerCase() === "src") {
          var v = String(value);
          var last = this.__ga4LastSrcDuplicated;
          if (v && v !== last) {
            duplicateIfGA4Url(v);
            this.__ga4LastSrcDuplicated = v;
          }
          // Also attach a load listener to capture final URL
          var selfAttr = this;
          var onloadOnceAttr = function () {
            try {
              var finalUrlAttr = selfAttr.src;
              if (
                finalUrlAttr &&
                finalUrlAttr !== selfAttr.__ga4LastSrcDuplicated
              ) {
                duplicateIfGA4Url(finalUrlAttr);
                selfAttr.__ga4LastSrcDuplicated = finalUrlAttr;
              }
            } catch (_) {}
            selfAttr.removeEventListener("load", onloadOnceAttr);
          };
          this.addEventListener("load", onloadOnceAttr);
        }
      } catch (_) {}
      return originalScriptSetAttribute.apply(this, arguments);
    };
  } catch (e) {
    // ignore environment where HTMLScriptElement may not be available
  }
})();</script>

4. Configure the Trigger

   • In the Triggering section, click to add a trigger
   • Select Initialization - All Pages
   • This ensures the code runs before any GA4 events fire

5. Set Up Tag Sequencing

   • In the tag configuration, expand Advanced Settings
   • Go to Tag Sequencing
   • Under "Setup Tag", select your main Google Tag (your GA4 configuration tag)
   • This ensures the GA4 tag loads first, then your duplicator runs

6. Save and Publish

   • Save your tag
   • Submit the changes and publish your container
   • Test thoroughly to ensure both GA4 and your custom endpoint receive events

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Method 2: Redirect All GA4 Requests

⚠️ Warning: This method will stop sending data to your GA4 property. Use only if you want to completely redirect tracking to your d8a instance.

Steps:

1. Open Google Tag Manager

   • Navigate to your GTM container
   • Go to the Tags section

2. Modify Your Google Tag

   • Find your existing Google Tag (GA4 Configuration tag)
   • Edit the tag configuration

3. Change the Server Container URL

   • In the tag Configuration settings, add a new parameter called server_container_url
   • Set its value to your endpoint URL, e.g. https://example.org/g/collect

---

Testing Your Setup

After implementing either method:

1. Open Browser DevTools

   • Go to Network tab
   • Filter by "collect" or your endpoint domain

2. Trigger Test Events

   • Navigate pages, click buttons, submit forms
   • Watch for outgoing requests

3. Verify Data Flow

   • Method 1: Check both GA4 and your endpoint receive data
   • Method 2: Check only your endpoint receives data

4. Use GTM Preview Mode

   • Enable Preview mode in GTM
   • Verify tags fire in the correct sequence
   • Check for any errors in the console