How to match by 'undefined' value in MongoDB Aggregation Framework?

Question

How can I search for the records filtering in a field that has an undefined value:

db.records.aggregate({
    $match: {
        myField: "undefined",
    }
})

Answer 1

Filter it by $type:6, (mongodb referece, note that this type marked as 'deprecated'):

db.records.aggregate({
    $match: {
        myField: {'$type':6},
    }
})