As we plan to eventually remove support of server-side javascript, one of the prerequisites for it is making sure users can easily rewrite their exising queries using js into native agg, so we may want to introduce some new expressions.
Today when js is used in query, in $queryStats output it's captures like

filter: { '$where': '?javascript' }

It would be very helpful to know which native js methods were used inside that javascript (e.g. JSON.stringify(), hex_md5() etc) for queries with $where, $function, $accumulator