Claude Code Skill for GTM JavaScript – No More ES5 Compilation Errors

December 2, 2025
{formatDate(new Date('2025-12-02'))}

The Problem

I've been working with Google Tag Manager for years, and every time I asked an AI assistant to write GTM code, the same thing happened: it would use const, arrow functions, or template literals – and GTM would reject it with compilation errors.

Error: This language feature is only supported for ECMASCRIPT_2015 mode or better: const declaration.

GTM's Custom HTML tags only support ES5 JavaScript. This isn't documented prominently, and most developers discover it the hard way.

I had to manually convert everything back to ES5, every single time. So I built a Claude Code skill to solve this problem once and for all.

The Solution

GitHub - claude-skill-gtm-javascript

This skill teaches Claude Code the GTM constraints:

Feature ES6+ (Prohibited) ES5 (Required)
Variables const, let var
Functions () => {} function() {}
Strings `${var}` 'str' + var
Destructuring {a, b} = obj var a = obj.a
for-of for (x of arr) for (var i...)

What's Included

The skill includes four comprehensive files:

SKILL.md

Core rules and patterns for ES5 code generation

reference.md

Complete ES6 → ES5 conversion reference

examples.md

Production-ready code for dataLayer, ecommerce, consent

checklist.md

Testing and debugging guide for GTM

2024-2025 Updates

The skill also includes the latest GTM/GA4 updates:

  • ✓ IE11 Support Ended (July 15, 2024) – No longer need IE11 compatibility
  • ✓ Consent Mode v2 Required (March 2024) – New ad_user_data, ad_personalization parameters
  • ✓ Google Ads Auto-Tag (April 10, 2025) – Containers auto-load Google tag first
  • ✓ GA4 Ecommerce – Complete event schemas with items array
  • ✓ Server-side GTM – 1st-party domain patterns

Installation

git clone https://github.com/ekusiadadus/claude-skill-gtm-javascript.git ~/.claude/skills/gtm-javascript

Or for project-specific installation:

mkdir -p .claude/skills
cp -r skills/gtm-javascript .claude/skills/

Example: GA4 Purchase Event

Once installed, Claude Code will generate proper ES5 code:

(function() {
  'use strict';


  window.dataLayer = window.dataLayer || [];
  window.dataLayer.push({ ecommerce: null });
  window.dataLayer.push({
    event: 'purchase',
    ecommerce: {
      transaction_id: 'T_12345',
      value: 99.99,
      currency: 'USD',
      items: [{
        item_id: 'SKU_001',
        item_name: 'Product Name',
        price: 99.99,
        quantity: 1
      }]
    }
  });
})();

Example: Consent Mode v2

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


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

Why This Matters

GTM runs code through Google Closure Compiler, which only supports ES5 for Custom HTML tags. This limitation isn't going away – it's been this way for years.

Every developer working with GTM faces this problem. Every AI assistant generates ES6+ code by default. This skill bridges that gap.

Feedback Welcome

I'd love to hear from anyone working with GTM or building Claude Code skills:

  • Are there GTM patterns I missed?
  • What other analytics tools have similar constraints?
  • How can the skill be improved?

GitHub Repository

© 2025 Daisuke Kuriyama