Go to editor, for example within Aggregations tab to create new aggregation pipeline.

Add any valid expression that accepts numeric values as value for expression (some code removed for brevity):

// code placeholder
/**
 * specifications - The fields to
 *   include or exclude.
 */
{
  _id: 0,
  max_height: {
    $reduce: {
      input: "validfield",
      initialValue: -Infinity,
      in: "validfield
    }
  }
}

The Compass editor does not allow to use Infinity/-Infinity as valid expression value and outputs:

Expected "[" or AggregationStage but "{" found.

The sample code is used on Chapter 3 of MongoDB University aggregation framework course