[{availableProducts:["mobile","laptop","desktop"],other_field:"an other value"}]

db.collection.aggregate([{$addFields:{"availableProducts":{$reduce:{input:"$availableProducts",initialValue:"",in:{"$cond":{if:{"$eq":[{"$indexOfArray":["$availableProducts","$$this"]},0]},then:{"$concat":["$$value","$$this"]},else:{"$concat":["$$value",",","$$this"]}}}}}}},])