GTM Variables: Reading Data from the Data Layer

What GTM Data Layer Variables Are

In GTM, a Data Layer Variable is a variable type that reads a specific key from the data layer. When a tag needs to include a value that came from a dataLayer.push (like an order ID or transaction value), you configure a Data Layer Variable pointing to that key, then reference the variable in the tag.

Creating a Data Layer Variable

1. GTM → Variables → User-Defined Variables → New
2. Variable Type: Data Layer Variable
3. Data Layer Variable Name: the key path you want to read
4. Data Layer Version: Version 2 (almost always)
5. Name it descriptively: DL - Transaction ID, DL - Ecommerce Value

Reading Top-Level Keys

For a push like:

dataLayer.push({
  'event': 'purchase',
  'transactionId': 'T12345',
  'transactionValue': 59.99
});

Data Layer Variable Name: transactionId → reads 'T12345'

Data Layer Variable Name: transactionValue → reads 59.99

Reading Nested Keys (Dot Notation)

For nested objects:

dataLayer.push({
  'event': 'purchase',
  'ecommerce': {
    'transaction_id': 'T12345',
    'value': 59.99,
    'items': [{'item_id': 'SKU-001'}]
  }
});

Data Layer Variable Name: ecommerce.transaction_id → reads 'T12345'

Data Layer Variable Name: ecommerce.value → reads 59.99

Data Layer Variable Name: ecommerce.items.0.item_id → reads the first item's ID

Default Values

Set a Default Value for the variable to return when the key is not present in the data layer — useful when the variable is used in trigger conditions (prevents undefined from causing errors):

• For string variables: default '' (empty string)
• For numeric variables: default 0
• For boolean variables: default false

Using Variables in Tags

In GA4 Event tags, reference data layer variables using double curly-brace syntax:

• Event Parameters → Add Parameter → Name: transaction_id, Value: {{DL - Transaction ID}}
• Event Parameters → Add Parameter → Name: value, Value: {{DL - Ecommerce Value}}

Variables in Trigger Conditions

Data Layer Variables can also be used in trigger conditions to restrict when a trigger fires. Example: only fire a purchase tag when the transaction value is above £50:

• Trigger: Custom Event "purchase"
• Fire this trigger when: DL - Ecommerce Value greater than 50

Summary

Data Layer Variables in GTM read specific keys from the data layer using the key name (top-level) or dot notation (nested). Configure them with descriptive names (prefix DL -) and set default values. Reference them in tag parameters with double curly braces. Data Layer Version 2 is the standard — Version 1 behaves differently and should not be used in new implementations.

See our GTM Data Layer Implementation service for variable configuration.

Need GTM data layer variables set up? Contact Adslytics.