OPTIONAL: If populated, runs a user script function and if return value is non-null doesn't create the object and logs the output. *Not* wrapped in $SCRIPT().

onUpdateScript

OPTIONAL: Used to preserve existing metadata when documents are updated, and also to generate new metadata based on the differences between old and new documents. *Not* wrapped in $SCRIPT().

OPTIONAL: A comma-separated list of top-level metadata fields to either exclude (if "metadataFields" starts with '-'), or only include (starts with '+', default) - the fields are deleted at that point in the pipeline.