The availability of large data sets represented as JSON documents is demanding for powerful tools able to integrate and query them. The J-CO Framework is a research prototype under development at University of Bergamo (Italy), specifically devised to provide practical tools to manage possibly large JSON data sets. Its query language, named J-CO-QL, provides constructs to evaluate the belonging of documents to fuzzy sets, so as to perform soft queries on JSON documents. In this paper, we present a real case study that shows how a novel construct, which enables defining JavaScript functions within J-CO-QL queries, provides users with an even improved possibility to personalize soft operators used to evaluate membership degrees.