JQuery Ajax fills javascript variable issue

Your backend code should look something more like this (WebService.cs in my example, which is the CodeBehind for WebService.asmx ):

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Services;

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class WebService : System.Web.Services.WebService
{
    public class JsonResult
    {
        public string key, label, open;
        public List<Children> children;
    }
    public class Children
    {
        public string key, label;
    }

    [WebMethod]
    public List<JsonResult> test()
    {
        List<Children> child = new List<Children>();
        child.Add(new Children
        {
            key = "211",
            label = "Burger"
        });

        List<JsonResult> result = new List<JsonResult>();
        result.Add(new JsonResult
        {
            key = "1",
            label = "Food",
            open = "false",
            children = child
        });
        return result;
    }
}

This code sends the created list as JSON - which looks like this when jQuery receives it:

{"d":[
    {
        "__type":"WebService+JsonResult",
        "key":"1",
        "label":"Food",
        "open":"false",
        "children":[
            {
                "key":"211",
                "label":"Burger"
            }
        ]
    }
]}

Then, the jQuery AJAX is very simple, like so:

$.ajax({
    type: "POST",
    url: "/WebService.asmx/test",
    contentType: "application/json",
    success: function (response) {
        var jsonResponse = response.d;
    }
});

Now you can access each element as native JavaScript objects, like I show here in the success callback when assigning each object to a variable. Now you can do anything you want to with the data.

success: function (response) {
    var jsonResponse = response.d;
    for (var i = 0; i < jsonResponse.length; i++) {
        var key = jsonResponse[i].key;
        var label = jsonResponse[i].label;
        var open = jsonResponse[i].open;
        var children = jsonResponse[i].children;
        for (var a = 0; a < children.length; a++) {
            var key = children[a].key;
            var label = children[a].label;
        }
    }
}

This is 100% tested and working. Let me know if there's anything else I can help you with.