Compare two object documents in MongoDB 4.4 with $function aggregation pipeline

aggregation.js

/*
Outputs: 
{
  "_id": ObjectId("5eec4fd0aee4bf9d2e209e2d"),
  "diff": {
    "haveMore": {
      "foo2": null
    },
    "types": {
      "differentType": NumberLong("55"),
      "objectIdDiff": ObjectId("5eec5020aee4bf9d2e209e2e"),
      "stringDiff": "different",
      "longDiff": NumberLong("99"),
      "decimalDiff": NumberDecimal("11"),
      "dateDiff": ISODate("2020-06-19T05:44:39.542Z")
    },
    "arrayDiff": [
      "x"
    ],
    "arrayDiffLength": [
      undefined,
      "b",
      "c"
    ],
    "arrayObjects": [
      undefined,
      {
        "xoo": null
      }
    ],
    "missingField": null
  }
}
*/
db.a.aggregate([
    {"$project":{
        "diff":{
            "$function":{
                "body": function(objectA, objectB) {
                    var diff = function (obj1, obj2) {
                        let type1 = Object.prototype.toString.call(obj1);
                        let type2 = Object.prototype.toString.call(obj2);
                        if (type2 === '[object Undefined]') {
                            return obj1;
                        }
                        if (type1 !== type2) {
                            return obj1;
                        }
                        if (type1 === '[object ObjectId]' || type1 === '[object NumberDecimal]') {
                            if (obj1.toString() !== obj2.toString()) {
                                return obj2;
                            }
                        }
                        if (type1 === "[object Date]" || type1 === "[object NumberLong]"){
                            if (obj1.valueOf() !== obj2.valueOf()) {
                                return obj2;
                            }
                        }
                        if (type1 === "[object String]") {
                            if (obj1 !== obj2) {
                                return obj2;
                            }
                        }
                        if (type1 === '[object BSON]') {
                            let result = {}
                            for (let key in obj1) {
                                if (obj2.hasOwnProperty(key)) {
                                    let tmp = diff(obj1[key], obj2[key]);
                                    if (tmp !== undefined){
                                        result[key] = tmp;
                                    }
                                } else {
                                    result[key] = null;
                                }
                            }
                            if (Object.keys(result).length > 0){
                                return result;
                            } else {
                                return undefined;
                            }
                        }
                        if (type1 === '[object Array]') {
                            let result = [];
                            for (let i = 0; i < obj1.length; i++) {
                                let tmp = diff(obj1[i], obj2[i]);
                                if (tmp !== undefined){
                                    result[i] = tmp;
                                }
                            }
                            if(result.length > 0){
                                return result;
                            } else {
                                return undefined;
                            }
                        }
                        return undefined;
                    };
                    return diff(objectA, objectB);
                }, 
                "args":[ {"$arrayElemAt": ["$docs", 0]}, {"$arrayElemAt":["$docs", 1]}], 
                "lang":"js"}
            }
    }}
]).pretty()

document.js

{
    _id: ObjectId("5eec4fd0aee4bf9d2e209e2d"), 
    "docs": [
        {
            "haveMore": {
                "foo1": "x1", 
                "foo2": "x2"
            },
            "haveLess": {
                "bar1": "x1"
            },
            "types": {
                "differentType": NumberLong("55"), 
                "objectIdEqual": ObjectId("5eec4f0aaee4bf9d2e209e2b"), 
                "objectIdDiff": ObjectId("5eec4f0aaee4bf9d2e209e2c"), 
                "stringEqual": "equal", 
                "stringDiff": "equal",
                "longEqual": NumberLong("11"), 
                "longDiff": NumberLong("11"), 
                "decimalEqual": NumberDecimal("99"),
                "decimalDiff": NumberDecimal("99"), 
                "dateEqual": ISODate("2020-06-19T05:36:21.995Z"), 
                "dateDiff": ISODate("2020-06-19T05:36:22.995Z"), 
            }, 
            "arrayEqual": ["a", "b"], 
            "arrayDiffLength": ["a", "b", "c"],
            "arrayObjects": [
                {"foo":{"bar": "baz"}}, 
                {"xoo":{"xar": "xaz"}}
            ], 
            "missingField": "x"
        }, 
        {
            "haveMore": {
                "foo1": "x1", 
            },
            "haveLess": {
                "bar1": "x1",
                "bar2": "x2"
            },
            "types": {
                "differentType": "55", 
                "objectIdEqual": ObjectId("5eec4f0aaee4bf9d2e209e2b"), 
                "objectIdDiff": ObjectId("5eec5020aee4bf9d2e209e2e"), 
                "stringEqual": "equal", 
                "stringDiff": "different",
                "longEqual": NumberLong("11"), 
                "longDiff": NumberLong("99"), 
                "decimalEqual": NumberDecimal("99"),
                "decimalDiff": NumberDecimal("11"), 
                "dateEqual": ISODate("2020-06-19T05:36:21.995Z"), 
                "dateDiff": ISODate("2020-06-19T05:44:39.542Z"), 
            }, 
            "arrayEqual": ["a", "b"], 
            "arrayDiffLength": ["a"], 
            "arrayObjects": [
                {"foo":{"bar": "baz"}}, 
                {"yoo":{"yar": "yaz"}}
            ], 
        }, 
    ]
}