Почему formData не является конструктором?

images_upload_handler: function (blobInfo, success, failure) {
        var xhr, formData;
        xhr = new XMLHttpRequest();
        xhr.withCredentials = false;
        xhr.open('POST', 'upload.php');
        xhr.onload = function() {
          var json;
    
          if (xhr.status != 200) {
            failure('HTTP Error: ' + xhr.status);
            return;
          }
          json = JSON.parse(xhr.responseText);
    
          if (!json || typeof json.location != 'string') {
            failure('Invalid JSON: ' + xhr.responseText);
            return;
          }
          success(json.location);
        };
        formData = new FormData();
        formData.append('file', blobInfo.blob(), fileName(blobInfo));
        xhr.send(formData);
      }