fix: wrapping logic in a conditional

Author: erunion

lib/dereference.ts

@@ -126,22 +126,26 @@ function crawl<S extends object = JSONSchema, O extends ParserOptions<S> = Parse
               // If we have properties we want to preserve from our dereferenced schema then we need
               // to copy them over to our new object.
               const preserved: Map<string, unknown> = new Map();
-              if (typeof obj[key] === "object" && !Array.isArray(obj[key])) {
-                (derefOptions?.preservedProperties || []).forEach((prop) => {
-                  if (prop in obj[key]) {
-                    preserved.set(prop, obj[key][prop]);
-                  }
-                });
+              if (derefOptions?.preservedProperties) {
+                if (typeof obj[key] === "object" && !Array.isArray(obj[key])) {
+                  derefOptions?.preservedProperties.forEach((prop) => {
+                    if (prop in obj[key]) {
+                      preserved.set(prop, obj[key][prop]);
+                    }
+                  });
+                }
               }
 
               obj[key] = dereferenced.value;
 
               // If we have data to preserve and our dereferenced object is still an object then
               // we need copy back our preserved data into our dereferenced schema.
-              if (preserved.size && typeof obj[key] === "object" && !Array.isArray(obj[key])) {
-                preserved.forEach((value, prop) => {
-                  obj[key][prop] = value;
-                });
+              if (derefOptions?.preservedProperties) {
+                if (preserved.size && typeof obj[key] === "object" && !Array.isArray(obj[key])) {
+                  preserved.forEach((value, prop) => {
+                    obj[key][prop] = value;
+                  });
+                }
               }
 
               derefOptions?.onDereference?.(value.$ref, obj[key], obj, key);