Decode JSON with jQuery / AJAX

{"Students":[{"Name":John,"Grade":17,}],"TotalClass":17,"TotalCount":1,}

is not valid JSON !

Assuming you have a valid JSON like this

{    "Students": [        {            "Name": "John",            "Grade": "17"        }    ],    "TotalClass": " 17",    "TotalCount": "1"}

You can access the values like this

alert("TotalClass : "+msg.TotalClass);//loop thru students$.each(msg.Students,function(index,item){   alert(item.Name+ " - "+item.Grade)}); 

Working sample : http://jsfiddle.net/ncbLF/5/

Use jsonlint to validate JSON

So your code can be simplified to

$.getJSON("class.aspx/getClass",function(msg){    alert("TotalClass : "+msg.TotalClass);    $.each(msg.Students,function(index,item){        alert(item.Name+ " - "+item.Grade)    });});

contentType is the type of the data sent to the server, not from. Remove that.

The JSON you included in the question. Is that the exact JSON the server returns? Because if it is, you don't need the $.each. You have an object, you should only need $.each to loop though an array of objects.

So, just try alert(msg.TotalClass).

Also, that JSON is invalid. You have an extra , after TotalCount, and after Grade. Also, John should be in double quotes.

Just try to alert

$j.each(msg, function (key, element) {    alert(key); // output: Students, TotalClass..    alert(element); //output: [{"Name":John,"Grade":17,}, 17..});

Note

as you set dataType: 'json' so I think you don't need any additional parse effort and given JSON has error, don't know is it your written or originally sent from server.

And you don't need the line

contentType: 'application/json; charset=utf-8',

You valid json should look like:

{    "Students": [        {            "Name": "John",            "Grade": "17"        }    ],    "TotalClass": " 17",    "TotalCount": "1"}