# Scripting

## Objective

Joule core provides the ability to deploy declarative expressions using the custom analytics processor.

This has been reused within the context of feature engineering to enable users to define custom calculations within the DSL.

## Example & DSL attributes

The example demonstrates per-event calculation of a `spend_ratio` using a JavaScript-based expression, where users can define variables, such as `avg_spend` to customise the calculation.

### Example

```yaml
feature engineering:
  ...
  features:
    compute:
      spend_ratio:
        scripting:
          macro:
            expression: 1 - spend/avg_spend
            variables:
              avg_spend: 133.78
```

### Attributes schema

<table><thead><tr><th width="155">Attribute</th><th width="358">Description</th><th width="133">Data Type</th><th data-type="checkbox">Required</th></tr></thead><tbody><tr><td>expression</td><td>A valid mathematical expression</td><td>String</td><td>true</td></tr><tr><td>language</td><td>List of supported feature functions</td><td><p>String</p><p>Default: js</p></td><td>false</td></tr><tr><td>variables</td><td>Map of variables to be used during the execution of the expression. </td><td>Map</td><td>false</td></tr></tbody></table>