Eval will return to the backend data analysis returned json object

// use eval json converting data transmitted over into JavaScript object
var data = eval ( '(' + request.responseText + ')'); // in brackets as a whole to resolve, did not increase the turn, then, is only data, not into the json object type, you can not obtain the corresponding data by a key.

// or used as follows
var obj = JSON.parse (jsonStr); // convert the JSON object