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Template Expressions

Loopstack uses two distinct expression syntaxes for dynamic content rendering and data access throughout your automation configurations.

Handlebars Expressions ({{ }})

Used for rendering dynamic content within text, typically in tool arguments, message content, and document templates. Template expressions are powered by Handlebars  and always evaluate to strings.

transitions:
    # ...
  - id: next
    from: signup_received
    to: user_email_notified
    call:
      - tool: EmailTool
        arguments:
          subject: "Welcome {{user.name}}!"
          body: |
            Hello {{user.name}},
 
            {{#if user.isPremium}}
              Thank you for being a premium member!
            {{else}}
              Consider upgrading to premium for more features.
            {{/if}}
 
            Best regards,
            The Team

Learn more about Template Expressions

JEXL Expressions (${{ }})

Used for evaluating expressions and accessing data in YAML configuration values. Powered by JEXL , these expressions support arithmetic, comparisons, ternaries, and more — while preserving the original data type.

transitions:
    # ...
  - id: next
    from: signup_received
    to: user_email_notified
    call:
      - tool: DataProcessingHandler
        arguments:
          user_id: ${{ ctx.workflow.user_id }}
          input_data: ${{ inputData }}                         # preserves data type (object, array, etc.)
          is_eligible: ${{ user.age >= 18 && user.isActive }}  # evaluates to boolean
          total: ${{ price - price * discount }}               # evaluates to number
          label: ${{ score > 90 ? "excellent" : "good" }}      # ternary expression

Learn more about JEXL Expressions

Key Differences

Both syntaxes can access the same underlying data, but serve different purposes:

  • Template expressions render dynamic text content with conditional logic and formatting (always outputs strings)
  • JEXL expressions evaluate full expressions and inject the result into configuration parameters while preserving the original data type (string, number, object, array, boolean, null, etc.)
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