This method will never be used with an extent and write = true

src/ol/format/feature.js

@@ -183,9 +183,9 @@ ol.format.Feature.transformWithOptions = function(
       // FIXME this is necessary because ol.format.GML treats extents
       // as geometries
       transformed = ol.proj.transformExtent(
-          write ? geometry.slice() : geometry,
-          write ? featureProjection : dataProjection,
-          write ? dataProjection : featureProjection);
+          geometry,
+          dataProjection,
+          featureProjection);
     }
   } else {
     transformed = geometry;
@@ -203,14 +203,10 @@ ol.format.Feature.transformWithOptions = function(
       }
       return coordinates;
     };
-    if (Array.isArray(transformed)) {
-      transform(transformed);
-    } else {
-      if (transformed === geometry) {
-        transformed = transformed.clone();
-      }
-      transformed.applyTransform(transform);
+    if (transformed === geometry) {
+      transformed = transformed.clone();
     }
+    transformed.applyTransform(transform);
   }
   return transformed;
 };