ajax.js

function saveToServer(nazwa) {
    $.ajax({
        type: "POST",
        url: window.location.href.replace("index.html", "") + "Server.aspx",
        data: {
            operation: "save",
            data: JSON.stringify({
                nazwa: nazwa,
                kolor: color,
                time: new Date(),
                trasa: JSON.stringify((function () {
                    var temp = []
                    for (var i = 0; i < markers.length; i++) {
                        var m = markers[i]
                        temp.push({
                            cords: m.cords,
                            name: m.name,
                            zoom: m.zoom
                        })
                    }
                    return temp;
                })())
            })
        },
        dataType: "text",
        success: function (r) {
            console.log(r)
        },
        error: function (err) {
            console.log("error " + err.responseText)
        }
    })
}

save.aspx.cs

void Save() {
        using (var sw = new StreamWriter(HttpContext.Current.Server.MapPath("plik.txt"), true, Encoding.UTF8)) {
            sw.WriteLine(Request["data"]+",");
        }
        Response.Write("save ok");
        Response.End();
    }