[{"field":5,"description":"a scalar number"},{"field":[5],"description":"an array with one scalar number"},{"field":[[5]],"description":"an array with one array"}]

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