var myString = "{Username:Brad,Password:12345}";

// adding in the quotes or else it throws an error saying 'unidentified token U
var myJson = JSON.parse('"' + myString + '"');

console.log(myJson.Username); // prints 'undefined'
console.log(typeof(myJson));  // prints 'string'

var myString = '{"Username":"Brad","Password":12345}';
var myJson = JSON.parse( myString );