How to Track JavaScript Errors in GTM and GA4 for Website Health Monitoring

Why Track JavaScript Errors? Most Marketers Miss This Critical Data

Most marketers obsess over tracking successes – conversions, goal completions, e-commerce sales. But what about tracking the problems people face on your website?

JavaScript errors can break forms, prevent purchases, and create terrible user experiences. Yet most businesses have no idea they’re happening.

The hidden cost of JavaScript errors:

Broken checkout processes (lost revenue)
Non-functioning contact forms (lost leads)
Poor user experience (higher bounce rates)
Site speed and performance degradation
SEO penalties from Core Web Vitals and indexing issues
Accessibility problems affecting compliance and user access
Search engine crawling and indexing failures
Support tickets that could be prevented

The solution: Track JavaScript errors in Google Analytics 4 using GTM so you can identify and fix problems before they cost you money.

What JavaScript Errors Look Like

Common JavaScript errors that break websites:

Uncaught TypeError: Cannot read property 'value' of null
ReferenceError: $ is not defined (jQuery issues)
TypeError: someFunction is not a function
Third-party script failures (chat widgets, analytics, etc.)
Browser compatibility issues

Real impact: A single JavaScript error can prevent your entire checkout process from working, slow down page loading, hurt your SEO rankings, and create accessibility barriers – and you might not know for weeks.

Setting Up JavaScript Error Tracking in GTM

Step 1: Create the JavaScript Error Trigger

In Google Tag Manager:

Go to Triggers → New
Trigger Name: Exception - JavaScript
Trigger Type: JavaScript Error
This trigger fires on: All JavaScript Errors
Save

What this does: This trigger activates whenever a JavaScript error occurs on any page of your website.

Step 2: Create the GA4 Event Tag

Create a new tag:

Tag Name: GA4 - Event - Exception (JS Error)
Tag Type: Google Analytics: GA4 Event
Measurement ID: {{GA4 - Measurement ID}} (use your constant variable)

Event Configuration:

Event Name: exception (keep lowercase)

Event Parameters: Add these four parameters:

Parameter 1:

Parameter: event_category
Value: JavaScript

Parameter 2:

Parameter: event_label
Value: {{Error Message}} (Built-in variable)

Parameter 3:

Parameter: exception_ref
Value: {{Error Line}} (Built-in variable)

Parameter 4:

Parameter: exception_source
Value: {{Error URL}} (Built-in variable)

Triggering:

Select the Exception - JavaScript trigger

Save the tag

Why these parameters:

event_category and event_label are reusable patterns from Universal Analytics
exception_ref shows which line of code caused the error
exception_source identifies which file or script is problematic

Setting Up Custom Dimensions in GA4

Important: GA4 won’t automatically create dimensions for your parameters. You must set them up manually.

In GA4:

Go to Admin → Data Display → Custom Definitions
Click Create Custom Dimension

Create these four custom dimensions:

Dimension 1: Event Category

Dimension Name: Event Category
Description: event_category
Scope: Event
Event Parameter: event_category

Dimension 2: Event Label

Dimension Name: Event Label
Description: event_label
Scope: Event
Event Parameter: event_label

Dimension 3: Exception Source

Dimension Name: Exception Source
Description: exception_source
Scope: Event
Event Parameter: exception_source

Dimension 4: Exception Reference

Dimension Name: Exception Ref
Description: exception_ref
Scope: Event
Event Parameter: exception_ref

Save each dimension

Testing Your JavaScript Error Tracking

Method 1: GTM Preview Mode

Activate Preview Mode in GTM
Navigate to your website
Look for JavaScript errors in the Tag Assistant
Verify the GA4 Event tag fires when errors occur

Method 2: Force a JavaScript Error (For Testing)

In browser console, type:

javascript

throw new Error("Test error for tracking");

This will trigger your error tracking so you can verify it’s working.

Method 3: Check Network Tab

In Chrome DevTools:

Go to Network tab
Look for collect requests to Google Analytics
Verify parameters are being sent correctly

Creating JavaScript Error Reports in GA4

Wait 24-48 hours for data collection before creating reports.

Step 1: Create a Custom Exploration

In GA4:

Go to Explore → Blank Exploration

Step 2: Configure Dimensions

Add these dimensions:

Page Path + Query String
Event Category
Event Label
Exception Ref
Exception Source

Add to Rows:

Event Label (the actual error message)
Page Path + Query String (where the error occurred)
Exception Ref (line number)
Exception Source (which file)

Step 3: Configure Metrics

Add metrics:

Event Count

Add to Values:

Event Count

Step 4: Apply Filters

Under Filters:

Event Category exactly matches JavaScript

Step 5: Analyze Your Data

Your report will show:

Which pages have the most JavaScript errors
What specific errors are occurring
Which files/scripts are causing problems
How frequently errors happen

Understanding Your JavaScript Error Data

SEO and Performance Impact

JavaScript errors affecting search rankings:

Core Web Vitals degradation – errors can block rendering and hurt performance scores
Failed lazy loading – images and content not loading properly for crawlers
Broken structured data – schema markup errors preventing rich snippets
Accessibility violations – screen reader compatibility issues from JS errors
Crawl budget waste – search engines spending time on broken pages

Performance implications:

Memory leaks from unhandled errors causing browser slowdowns
Render-blocking errors preventing above-the-fold content loading
Third-party script failures cascading to affect site performance
Mobile performance hits – errors often worse on slower devices

Common Error Patterns to Watch For

Form submission problems
Checkout process issues
Interactive element failures

Errors from third-party scripts:

Chat widgets loading incorrectly
Social media embeds failing
Advertising scripts causing conflicts

Browser-specific errors:

IE/Edge compatibility issues
Mobile browser problems
Different JavaScript engine behaviors

Priority Levels for Fixing Errors

Critical (Fix Immediately):

Errors on checkout or form pages
Errors affecting conversion tracking
Errors preventing core functionality
Performance-blocking errors affecting Core Web Vitals
Accessibility-breaking errors affecting screen readers

High Priority:

Errors on high-traffic pages
Errors affecting user experience
Recurring errors across multiple pages
SEO-impacting errors (structured data, lazy loading)
Mobile-specific performance errors

Medium Priority:

Errors from third-party scripts
Errors on low-traffic pages
Minor cosmetic JavaScript issues

Advanced Error Tracking Configurations

Track Errors by User Type

Add user engagement parameters:

Parameter: user_type
Value: {{User Type Variable}}

Track Errors by Device

Add device information:

Parameter: device_category  
Value: {{Device Category}}

Set Up Error Alerts

Create custom alerts in GA4:

Go to Admin → Custom Insights
Set up alerts for JavaScript error spikes
Get notified when critical pages have errors

Business Impact of JavaScript Error Tracking

Real-World Examples

E-commerce site: Discovered checkout JavaScript errors were causing 15% cart abandonment – fixed and recovered $50k/month in lost sales.

Lead generation site: Found contact form errors were preventing 25% of submissions – quick fix increased leads by 400%.

Content site: JavaScript errors were breaking ad scripts, causing 30% revenue loss – tracking identified the problem in 24 hours instead of months.

SEO case study: E-commerce site had JavaScript errors preventing product schema markup from loading – lost rich snippets and 40% drop in organic traffic until errors were fixed.

Performance case study: News site had JavaScript errors causing memory leaks – page speed dropped from 85 to 45 Core Web Vitals score, hurting search rankings until tracking identified the specific script causing issues.

ROI of Error Tracking

Time to identify issues:

Without tracking: Weeks or months (if ever)
With tracking: Hours or days

Cost of delayed fixes:

Lost conversions during error periods
Reduced user trust and experience
SEO penalties from poor Core Web Vitals and crawling issues
Accessibility compliance violations and potential legal issues
Organic traffic loss from search ranking drops
Support costs from confused users

Next Steps: Expanding Your Error Tracking

This JavaScript error tracking provides the foundation for comprehensive website health monitoring.

Related error tracking opportunities:

404 Error Tracking in WordPress (upcoming post)
Form submission errors
Image loading failures
API call errors

Professional website health monitoring:

Automated error alerting systems
Cross-browser error analysis
Performance impact assessment
Integration with development workflows

Professional Implementation Services

While this tutorial covers JavaScript error tracking basics, comprehensive website health monitoring often requires additional expertise:

Advanced error tracking scenarios:

Multi-site error monitoring and reporting
Integration with development and QA workflows
Custom error categorization and prioritization
Automated alerting and response systems
Performance impact analysis

When to get professional help:

Complex e-commerce or application websites
High-traffic sites where errors cost significant revenue
Teams needing training on error analysis and response
Integration with existing monitoring and alerting systems

Need help implementing comprehensive website health monitoring? Contact Knihter for professional GTM implementation and website optimization services. We specialize in identifying and fixing issues that impact conversions and user experience.

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