A an example of a JS Block is provided along with the descriptors on the right hand side.
Use the Raw descriptor field to define the configuration options for block (see Descriptor)
Use the Raw event descriptor field to define any events that your block will trigger.
Iterating Data Source
Iterating data source allows you to specify a field from your Block’s Descriptor that is of type “feed”. If specified, the Body field of the HTML Template will be repeated for each record in the data source selected in the block configuration for that feed field.
Under HTML Template, there are 3 fields: Prologue, Body and Epilogue. If nothing is entered for Iterating data source, these fields are simply concatenated in the order they appear. If an Iterating data source is configured, the Body section will be repeated for reach record in the data source that the user of the block configures for the Iterating data source field.