jQuery ajax method does not return data

Going by the jQuery docs for the call you’re using, it looks like maybe you want the success callback instead of complete. success gets passed the JSON directly, as you expect, but complete gets passed different stuff that you would have to look inside. Refer to the docs for more if you’re sure you want to use complete

Try success like this:

$.ajax({
        type: "POST",
        url: "/mailer.php",
        data: "from_name="+from_name_temp+"&from_email="+from_email_temp+"&choix_gout="+choix_gout_temp,
        dataType: "json",
        success: function(data) {
                console.log("Thank you for subscribing!");
                console.log(data);
                console.log(data.status);
                console.log(data.message);
                console.log(data.success);
            }
    });