1function isJson(str) {
2    try {
3        let value = JSON.parse(str);
4      	return value
5    } catch (e) {
6        return str;
7    }
8}
9

1var jsonPerson = '{"first_name":"billy", "age":23}';
2var personObject = JSON.parse(jsonPerson); //parse json string into JS object

1    var str = '[{"UserName":"xxx","Rolename":"yyy"}]'; // your response in a string
2    var parsed = JSON.parse(str); // an *array* that contains the user
3    var user = parsed[0];         // a simple user
4    console.log(user.UserName);   // you'll get xxx
5    console.log(user.Rolename);   // you'll get yyy