[{"healthDetails":[{"problem":"Fever","note":"normal"},{"problem":"Allergy","note":"critical"}]}]

db.collection.aggregate([{$set:{"healthDetails":{$trim:{input:{$reduce:{input:"$healthDetails",initialValue:"",in:{$concat:["$$value",", ",{$concat:["$$this.problem"," - ","$$this.note"]}]}}},chars:", "}}}}])