[{"data":"collection",}]

db.collection.aggregate([{$addFields:{data:{$reduce:{input:{$map:{input:["a","b","c"],as:"abc",in:{$let:{vars:{someObject:{$filter:{input:{$objectToArray:{a:"hello ",b:"world",c:"!"}},cond:{$eq:["$$this.k","$$abc"]}}}},in:{$ifNull:[{$arrayElemAt:["$$someObject.v",0]},""]}}}}},initialValue:"",in:{$concat:["$$value","$$this"]}}}}}])