[{"field":[42],"description":"an array with one scalar number"}]

db.collection.aggregate([{$project:{field:1,"field description":1," $isArray: [💬]":{$isArray:["💬"]}," is 42 a number?":{$isNumber:42}," is argument in [42] an array?":{$isArray:[5]}," is argument in [[42]] an array?":{$isArray:[[5]]}," is literal [42] an array?":{$isArray:{$literal:[42]}},"is field a number?":{$isNumber:"$field"},"is field an array?":{$isArray:"$field"}}}])