fix: transformResponse running twice on cached requests (#775)

src/interceptors/request.ts

@@ -219,6 +219,11 @@ export function defaultRequestInterceptor(axios: AxiosCacheInstance) {
       cachedResponse = cache.data;
     }
 
+    // The cached data is already transformed after receiving the response from the server.
+    // Reapplying the transformation on the transformed data will have an unintended effect.
+    // Since the cached data is already in the desired format, there is no need to apply the transformation function again.
+    config.transformResponse = undefined;
+
     // Even though the response interceptor receives this one from here,
     // it has been configured to ignore cached responses = true
     config.adapter = function cachedAdapter(): Promise<CacheAxiosResponse> {

test/interceptors/request.test.ts

@@ -354,6 +354,25 @@ describe('Request Interceptor', () => {
     assert.equal(headers2[Header.Expires], undefined);
   });
 
+  it('ensure cached data is not transformed', async () => {
+    const axios = mockAxios();
+
+    // data will transformed with first request
+    const res1 = await axios.get('url', {
+      transformResponse: (data: unknown) => [data]
+    });
+
+    assert.notEqual(res1.config.transformResponse, undefined);
+
+    // cached data should not transform the data as it is alread in desired format.
+    // transform function is nullified in this scenario
+    const res2 = await axios.get('url', {
+      transformResponse: (data: unknown) => [data]
+    });
+
+    assert.equal(res2.config.transformResponse, undefined);
+  });
+
   it('ensures request with urls in exclude.paths are not cached', async () => {
     const axios = mockAxios({
       cachePredicate: {

test/interceptors/response.test.ts

@@ -273,13 +273,19 @@ describe('Response Interceptor', () => {
   it('Works when modifying response', async () => {
     const axios = mockAxios();
 
-    const normal = await axios.get('url');
-    const transformed = await axios.get('url', {
+    // fresh response from server and transformed
+    const freshResponse = await axios.get('url', {
       transformResponse: (data: unknown) => [data]
     });
 
-    assert.ok(normal.data);
-    assert.deepEqual(transformed.data, [true]);
+    // cached response
+    // should not transform again as already in desired format
+    const cachedResponse = await axios.get('url', {
+      transformResponse: (data: unknown) => [data]
+    });
+
+    assert.deepEqual(freshResponse.data, [true]);
+    assert.deepEqual(cachedResponse.data, [true]);
   });
 
   it('Works when modifying the error response', async () => {