MongoDB: $function operator does not support arrow function

Question

I have following documents in testCollection:

[
  {
    "_id": ObjectId("60562f98d171d52ef0a5bb27"),
    "popularDate": ISODate("1947-08-15T00:00:00.000+05:30")
  },
  {
    "_id": ObjectId("60562f98d171d52ef0a5bb28"),
    "popularDate": ISODate("1950-01-26T00:00:00.000+05:30")
  },
  {
    "_id": ObjectId("60562f98d171d52ef0a5bb29"),
    "popularDate": ISODate("1994-01-15T00:00:00.000+05:30")
  }
]

I am using regular expression to filter documents using $function operator. I get the correct output while using Query 1.

Query 1:

let yearRegex = /^1947/;

db.testCollection.find({
    $expr: {
        $function: {
            body: function(popularDates, yearRegex) {
                return yearRegex.test(popularDates)
            },
            args: [{ $toString: "$popularDates" }, yearRegex],
            lang: "js"
        }
    }
});

Output for Query 1:

{
    "_id" : ObjectId("60562f98d171d52ef0a5bb27"),
    "popularDate" : ISODate("1947-08-15T00:00:00.000+05:30")
}

but for Query 2 I am getting all the documents and the filter is not working. In Query 2 I changed the function body to arrow function.

Query 2:

let yearRegex = /^1947/;

db.testCollection.find({
    $expr: {
        $function: {
            body: (popularDate, yearRegex) => yearRegex.test(popularDate),
            args: [{ $toString: "$popularDate" }, yearRegex],
            lang: "js"
        }
    }
});

Output for Query 2:

{
  "_id": ObjectId("60562f98d171d52ef0a5bb27"),
  "popularDate": ISODate("1947-08-15T00:00:00.000+05:30")
},
{
  "_id": ObjectId("60562f98d171d52ef0a5bb28"),
  "popularDate": ISODate("1950-01-26T00:00:00.000+05:30")
},
{
  "_id": ObjectId("60562f98d171d52ef0a5bb29"),
  "popularDate": ISODate("1994-01-15T00:00:00.000+05:30")
}

So now my question is why is arrow function not working inside $function operator, or am I missing something.

Answer 1

MongoDB relies on using javascript call to set this to the current document when calling the passed function.

Arrow functions do not have bindings to this or super (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions), so they don't work right in server-side javascript in MongoDB.