Django csrf token for Ajax

The documentation very well explained how to use AJAX https://docs.djangoproject.com/en/2.1/ref/csrf/

1. Get this library https://github.com/js-cookie/js-cookie/
2. Add this var csrftoken = Cookies.get('csrftoken');

3. The last step is configure ajax setup

   function csrfSafeMethod(method) {
   // these HTTP methods do not require CSRF protection
    return (/^(GET|HEAD|OPTIONS|TRACE)$/.test(method));
   }
   $.ajaxSetup({
     beforeSend: function(xhr, settings) {
       if (!csrfSafeMethod(settings.type) && !this.crossDomain) {
           xhr.setRequestHeader("X-CSRFToken", csrftoken);
       }
     }
   });
   

See below for how I changed your code. The csrf_token is assigned to a variable with Django templating. You can produce this variable in any of your Javascript code.

The token is then included in the header

 <script>
    var token = '{{csrf_token}}';

    $("#id_username").change(function () {
      console.log($(this).val());
      var form = $(this).closest("form");
      $.ajax({
        headers: { "X-CSRFToken": token },
        url: form.attr("data-validate-username-url"),
        data: form.serialize(),
        dataType: 'json',
        success: function (data) {
          if (data.is_taken) {
            alert(data.error_message);
          }
        }
      });

    });
  </script>