JS 'Set' type does not serialize well to BSON

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    • Type: Improvement
    • Resolution: Unresolved
    • Priority: Major - P3
    • None
    • Affects Version/s: None
    • Component/s: JavaScript
    • Query Optimization
    • None
    • 3
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      As you can see below - it just happens to return an empty object, but proceeds without error. It seems better to either (a) error or (b) return an array. We don't have a 'set' data type in the BSON spec, so I don't see how we could get it to work and round trip seamlessly, but this experience isn't great.

      > db.foo.aggregate([{
    $project: {
        js_res: {
            $function: {
                lang: "js",
                body: function() {
                    return new Set([1, 2]);
                },
                args: []
            }
        }
    }
}])
{ "_id" : ObjectId("5f3c24f10efd1cb7f0a88f4a"), "js_res" : {  } }
{ "_id" : ObjectId("5f3c24f60efd1cb7f0a88f4b"), "js_res" : {  } }
{ "_id" : ObjectId("5f47f9411945d1527c6b0368"), "js_res" : {  } }
{ "_id" : ObjectId("5f47f9c71945d1527c6b0369"), "js_res" : {  } }
{ "_id" : ObjectId("5f47fa721945d1527c6b036a"), "js_res" : {  } }

      As a workaround, you can use the [...set] syntax to expand the set into an array:

      > db.foo.aggregate([{
    $project: {
        js_res: {
            $function: {
                lang: "js",
                body: function() {
                    return [...new Set([1, 2])];
                },
                args: []
            }
        }
    }
}])
{ "_id" : ObjectId("5f3c24f10efd1cb7f0a88f4a"), "js_res" : [ 1, 2 ] }
{ "_id" : ObjectId("5f3c24f60efd1cb7f0a88f4b"), "js_res" : [ 1, 2 ] }
{ "_id" : ObjectId("5f47f9411945d1527c6b0368"), "js_res" : [ 1, 2 ] }
{ "_id" : ObjectId("5f47f9c71945d1527c6b0369"), "js_res" : [ 1, 2 ] }
{ "_id" : ObjectId("5f47fa721945d1527c6b036a"), "js_res" : [ 1, 2 ] }

            Assignee:
            [DO NOT USE] Backlog - Query Optimization
            Reporter:
            Charlie Swanson
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            Watchers:
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              Created:
              Updated: