JSON Cookie Reader (GTM Variable Template)

The JSON Cookie Reader is a Google Tag Manager (GTM) Custom Variable Template that reads a specified cookie, parses it as JSON, and returns the value from a given key path (e.g. utm_source, click_id.value).

It is especially useful when paired with a tag like the Attribution Cookie Writer, which stores attribution data in a JSON-formatted cookie.

Built by Daniel Perry-Reed – marketing analytics specialist at Data to Value.

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⚙️ What It Does

• Reads a cookie by name (e.g. gtm_attr)
• Decodes it (if URL-encoded)
• Parses the value as a JSON object
• Retrieves the value at a dot-notated key path (e.g. click_id.value)

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🚀 How to Use

📦 Option 1: Import the Prebuilt GTM Container

If you want a ready-to-go setup, use the provided GTM container that includes the JSON Cookie Reader template and an example variable.

1. Download the container file 📥 Download gtm_json_cookie_reader_container.json here

2. Import into GTM

   • Go to your GTM workspace > Admin > Import Container
   • Upload the downloaded JSON file
   • Choose your workspace
   • Select Merge and resolve any conflicts

3. Review and publish

   • The variable template and example variables should be ready to use
   • Update field values as needed and publish your container

🛠️ Option 2: Manual Setup

1. Add the Custom Variable Template

• In GTM, go to Templates > New Variable Template
• Paste in the JavaScript from json_cookie_reader.js here
• Configure the permissions (see below)
• Save and name your variable template (e.g. "JSON Cookie Reader")

2. Set Required Permissions

{
  "permissions": {
    "read_cookies": ["*"],
    "apiAccess": [
      "getCookieValues",
      "decodeUriComponent",
      "JSON"
    ]
  }
}

3. Create a Variable Using the Template

• Go to Variables > New

• Choose your custom variable template

• Fill in the fields:

  • Cookie name: the cookie to read from (e.g. gtm_attr)
  • Key path: the JSON key to extract (e.g. utm_source or click_id.type)

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🔍 Example Usage

For a cookie named gtm_attr containing:

{
  "utm_source": "google",
  "click_id": {
    "type": "gclid",
    "value": "abc123"
  }
}

Key Path	Returned Value
utm_source	google
click_id.type	gclid
click_id.value	abc123

If the cookie is missing or the key doesn’t exist, the variable will return an empty string.

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🟦 About Data to Value

Built and maintained by the team at Data to Value — your data activation partner helping marketing teams transform data into predictable revenue and growth.

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📄 License

MIT