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name: gtm-javascript description: Generate ES5-compliant JavaScript for Google Tag Manager Custom HTML tags. Use when writing GTM tags, dataLayer code, or analytics implementations.

GTM JavaScript Coding Standards

This skill ensures all JavaScript code generated for Google Tag Manager (GTM) Custom HTML tags is ES5-compliant and follows current best practices (2024-2025).

Critical Constraint: ES5 Only

GTM's JavaScript compiler operates in ES5 (ECMAScript 5) mode by default. ES6+ syntax causes compilation errors and prevents tag publishing.

Prohibited ES6+ Features

NEVER use these in GTM Custom HTML tags:

2024-2025 Updates

Breaking Changes

• IE11 Support Ended (July 15, 2024): No longer tested or fixed
• Consent Mode v2 Required (March 2024): New parameters ad_user_data and ad_personalization
• Google Ads Auto-Tag (April 10, 2025): Containers with Google Ads tags auto-load Google tag first

New Features

• Tag Diagnostics tool for issue detection
• gtagSet API for configuration settings
• readAnalyticsStorage sandbox API for custom templates
• Server-side GTM can load scripts via 1st-party domain

Code Patterns

IIFE Pattern (Recommended)

Copy(function() {
  'use strict';

  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({
    event: 'my_event',
    my_parameter: 'value'
  });
})();

Error Handling

Copy(function() {
  'use strict';
  try {
    window.dataLayer = window.dataLayer || [];
    window.dataLayer.push({event: 'tracked_event'});
  } catch (e) {
    // Silent fail - do not break page
  }
})();

Array Iteration (ES5)

Copyfunction forEach(arr, callback) {
  for (var i = 0; i < arr.length; i++) {
    callback(arr[i], i);
  }
}

Consent Mode v2

When implementing consent, use these parameters:

Copywindow.dataLayer = window.dataLayer || [];
function gtag() { dataLayer.push(arguments); }

// Default state (before consent)
gtag('consent', 'default', {
  ad_storage: 'denied',
  ad_user_data: 'denied',
  ad_personalization: 'denied',
  analytics_storage: 'denied'
});

// After user grants consent
gtag('consent', 'update', {
  ad_storage: 'granted',
  ad_user_data: 'granted',
  ad_personalization: 'granted',
  analytics_storage: 'granted'
});

GA4 Ecommerce Events

Use the standard event names and items array structure:

Copywindow.dataLayer = window.dataLayer || [];
window.dataLayer.push({
  event: 'purchase',
  ecommerce: {
    transaction_id: 'T12345',
    value: 99.99,
    currency: 'USD',
    items: [{
      item_id: 'SKU123',
      item_name: 'Product Name',
      price: 99.99,
      quantity: 1
    }]
  }
});

Validation Checklist

Before publishing any GTM tag:

1. No const/let - use var only
2. No arrow functions - use function() syntax
3. No template literals - use string concatenation
4. No destructuring - access properties individually
5. No for-of loops - use traditional for loops
6. No block-scoped function declarations
7. Test in GTM Preview Mode
8. Verify in GA4 DebugView (if applicable)
9. Check browser console for errors

Resources

• GTM Developer Guide
• GA4 Ecommerce
• Consent Mode
• Sandboxed JavaScript APIs

When to Use Custom Templates Instead

Consider using Custom Templates (not Custom HTML) when:

• Building reusable tag logic
• Need sandboxed security
• Want permission-based access control
• Sharing with organization

Custom Templates support some ES6 features and provide better security through the sandboxed JavaScript environment.