Saving date object in the correct format from Scala JS

I have a serverless application written in Scala.js that is writing to a MongoDb database. We wrote our own custom integration with the Node.js mongodb driver. Everything is working well except for date objects. I am attempting to find a way to serialize my objects so that mongo can interpret them as a date object, rather than a string.

Without getting to much in detail, the code uses Circe Encoders to serialize the objects to Json. Here is the encoder I am using for the date object:

  implicit val dateEncoder: Encoder[Date] = Encoder.encodeJson.contramap[Date] {
    date =>
        "$date" -> Json.fromString(date.toISOString())

The Circe Json then gets converted to Scala.js Json before interacting with the Node.js driver.

In the encoder above, I am attempting to replicate the bson format of date objects that I see in this documentation. Conceptually, it is working correctly, as I see the object is being saved with this value:

    "_id" : "test-id",
    "profile" : {
        "IndividualProfile" : {
            "username" : "",
            "dateCreated" : {
                "$date" : "2024-06-17T15:50:12.059Z"
            "dateUpdated" : {
                "$date" : "2024-06-17T15:50:12.059Z"

You can see the dateCreated and dateUpdated objects in the result, and they appear to have the json formatted as intended, but mongo is not interpreting the field as a date object: it’s just an object with a nested string field.
I get this is probably a nice use case, but I was curious if anyone had an idea what I might be missing here?

Fyi, for anyone curious, I was able to resolve this issue using bson’s EJSON.serialize() function. So I have my encoders construct the appropriate format (with the $date nested field), and then I call EJSON.serialize() on it, which converts it to the appropriate bson object. Once this is saved, MongoDb is able to interpret it correctly.

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