# js

Run `function()` on each event.  If the function returns a truthy value, it is emitted downstream.  Please [read these notes](/features/adhoc-code/#this-context) for details on arrow functions and `this`.

## Config

**Required:**
- `function`: Function to apply to events.

## Example

```yaml
pipeline:
  not-aggregated:
    module: js
    config:
      function: !!js/function >-
        function(event) {
          if (event._agg === undefined) {
            return event
          }
        }
```