var str = '[{"UserName":"xxx","Rolename":"yyy"}]'; // your response in a string

    var parsed = JSON.parse(str); // an *array* that contains the user

    var user = parsed[0];         // a simple user

    console.log(user.UserName);   // you'll get xxx

    console.log(user.Rolename);   // you'll get yyy

var jsonPerson = '{"first_name":"billy", "age":23}';

var personObject = JSON.parse(jsonPerson); //parse json string into JS object

//JSON.parse()

const json = "{\"name\": \"Example\", \"age\": 50}";

let parsed_json = JSON.parse(json);

console.log(`Name: ${parsed_json.name}; Age: ${parsed_json.age}`)

// Returns "Name: Example; Age: 50"

The JSON.parse() method parses a JSON string, constructing the JavaScript value or object described by the string. An optional reviver function can be provided to perform a transformation on the resulting object before it is returned.

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JSON.parse(data)

const json = '{"fruit": "apple", "mango": 10}'

const json = JSON.parse(json);

console.log(obj.fruit,obj.mango)