How to add dynamically calculated field to resultset in MongoDB? [duplicate]

How to add dynamically calculated filed/column to the result-set documents? I don't need aggregation but if that's the only way with Mongo... yet I'm having troubles with $project aggregation stage. How to pass filed value(s) to a (non-aggregating) JS function in $project stage?

I'm looking for a way to transform the following to MongoDB query:

SELECT CountryName, MD5(CountryName) as md5 FROM Country

I've tried aggregation

db.getCollection('Country').aggregate([
{ "$project":{ a: "$Abbreviation", b: hex_md5("$Abbreviation")} }
])

Expected:

{
    "_id" : 1.0,
    "a" : "US",
    "b" : "7516FD43ADAA5E0B8A65A672C39845D2"
},
{
    "_id" : 2.0,
    "a" : "JP",
    "b" : "24D22E03AFB23EDB45C6C8CFA16A280E"
}
// but I get "$Abbreviation" treated as a string instead - not replaced by actual field value(s)
// MD5("$Abbreviation") == 668eddfec9d4b8a3bb099aca67b365d8
{
    "_id" : 1.0,
    "a" : "US",
    "b" : "668eddfec9d4b8a3bb099aca67b365d8"
},
{
    "_id" : 2.0,
    "a" : "JP",
    "b" : "668eddfec9d4b8a3bb099aca67b365d8"
}