this.FuncName = function (variable) {
$.ajax(
                {
                    type: "POST",
                    url: '/.../...',
                    data: function(variable) {
                        var obj = { field1: null, field2: null };
                        if (variable) {
                obj.field1 = value1;
                                obj.field2 = value2;
            } else {
                obj.field1 = value3;
                                obj.field2 = value4;
            }
                        JSON.stringify(obj);
                    },
            ...
        }