JavaScript Processor is available from the Processor component in Composite DataSources.
The JavaScript Processor lets you perform any kind of record operation or schema re-arrangement which might be too complicated for regular flows.
This sample (TestProcessor.ds) takes the FruitSales.ds and turns the records into an across-then-down summary. This is useful for certain kinds of reports and is hard to do just using flows.
Note that the number of output records is less than the number of input records.
Another example (TestProcessor2.ds) generates more output records than the input records, based on the same set of data.
JavaScript saves a lot of complex joins, filters and concats to do the same thing.
FruitSales.zip