React native – How to convert the string to JSON in javascript

when i fetch the data from firebase real time database, i am getting string object like below. May i know how can i convert this to JSON object?. how can i get the data like Address:"Add line 1, Add line 2, Street,City,State.". ?. This is for my react native app.

What i tried:

• when i tried to use JSON.parse, i am getting the error ‘JSON Parse
  error: Unexpected token: o’.
• when i display the data, it is [object object]
• when i try typeof, i am getting result as object

data that i get from Firebase real time database(It is getting displayed like below when i use JSON.stringify):

user:{"Address":"Add line 1, Add line 2, Street,City,State.","Age":"42","CAppoint":{"0":{"AppointDate":"","ConsultDoc":"","Patient":"C1","Slot":"","Treatment":"","extra1":"","extra2":""}},"PAppoint":{"0":{"AppointDate":"","ConsultDoc":"","Patient":"P1","Slot":"","Treatment":"","extra1":"","extra2":""},"1":{"AppointDate":"","ConsultDoc":"","Patient":"P2","Slot":"","Treatment":"","extra1":"","extra2":""},"2":{"AppointDate":"","ConsultDoc":"","Patient":"P3","Slot":"","Treatment":"","extra1":"","extra2":""},"3":{"AppointDate":"","ConsultDoc":"","Patient":"P4","Slot":"","Treatment":"","extra1":"","extra2":""}},"name":"Zulu","pass":"1234","phone":"+918888888888"}

how i fetch data from firebase:

const readUserData = async (phoneNumber) => {
try {
      const reference = ref(db, 'x123456/' + phoneNumber);
      return (await get(reference)).toJSON((snapshot) => {
          const data = snapshot.val();
          console.log('DATA: ' + data);
      }, (error) => {
          console.error(error);
          return 0;
      });
}

How i call the above function:

      exist=await readUserData('+91'+phoneNumber);
      console.log('user:'+ JSON.stringify(exist));
    } catch (error) {
      alert(error);
    }```