Hi I am a beginner to Laravel and AJAX programming, and I am trying to send a JavaScript variable through the data field

Here is my code:

        $.ajax({
                type: "post",
                url: "/checklists/" + checkButtonId,
                data: {relevantClasses : JSON.stringify(relevantClasses)},
                dataType: "json",
                success: function (response) {
                    if(response.success) {
                        $(taskId).removeClass(relevantClasses);

                        alert(JSON.stringify(response.checked + " " + response.request + " " + response.id));

                        switch(response.checked) {
                            case 0:
                            $(taskId).addClass("far fa-square");
                            break;

                            case 1:
                            $(taskId).addClass("fas fa-check-square");
                            break;
                        }
                    }
                }
            });

The variable relevantClasses is a JavaScript let variable. I tried to stringify it with JSON and it returns in the alert, but in the Controller I can't compare this variable with a PHP one. (I used json_decode() function and nothing happens)

can you please help me?