feat: add served from response cache symbol to response cache (#3164)

.changeset/big-tables-clean.md

@@ -0,0 +1,7 @@
+---
+'@graphql-yoga/plugin-response-cache': minor
+---
+
+Add `servedFromResponseCache` symbol property to responses served from the response cache in order
+to allow other plugins to determine, whether a response was served from the cache and apply custom
+logic based on that.

packages/plugins/response-cache/__tests__/response-cache.spec.ts

@@ -1152,3 +1152,66 @@ describe('shouldCacheResult', () => {
     });
   });
 });
+
+it('response has "servedFromResponseCache" symbol', async () => {
+  const results: Array<any> = [];
+  const yoga = createYoga({
+    plugins: [
+      useResponseCache({
+        session: () => null,
+        includeExtensionMetadata: true,
+      }),
+      {
+        onResultProcess(context) {
+          results.push(context.result);
+        },
+      },
+    ],
+    schema,
+  });
+  function fetch() {
+    return yoga.fetch('http://localhost:3000/graphql', {
+      method: 'POST',
+      headers: {
+        'content-type': 'application/json',
+      },
+      body: JSON.stringify({ query: '{__typename}' }),
+    });
+  }
+
+  let response = await fetch();
+
+  expect(response.status).toEqual(200);
+  let body = await response.json();
+  expect(body).toEqual({
+    data: {
+      __typename: 'Query',
+    },
+    extensions: {
+      responseCache: {
+        didCache: true,
+        hit: false,
+        ttl: null,
+      },
+    },
+  });
+
+  response = await fetch();
+  expect(response.status).toEqual(200);
+  body = await response.json();
+  expect(body).toMatchObject({
+    data: {
+      __typename: 'Query',
+    },
+    extensions: {
+      responseCache: {
+        hit: true,
+      },
+    },
+  });
+
+  const [result1, result2] = results;
+  expect(Symbol.for('servedFromResponseCache') in result1).toEqual(false);
+  expect(Symbol.for('servedFromResponseCache') in result2).toEqual(true);
+  expect(result2[Symbol.for('servedFromResponseCache')]).toEqual(true);
+});

packages/plugins/response-cache/src/index.ts

@@ -170,10 +170,14 @@ export function useResponseCache(options: UseResponseCacheParameter): Plugin {
       if (enabled(request)) {
         const cachedResponse = await cache.get(operationId);
         if (cachedResponse) {
+          const responseWithSymbol = {
+            ...cachedResponse,
+            [Symbol.for('servedFromResponseCache')]: true,
+          };
           if (options.includeExtensionMetadata) {
-            setResult(resultWithMetadata(cachedResponse, { hit: true }));
+            setResult(resultWithMetadata(responseWithSymbol, { hit: true }));
           } else {
-            setResult(cachedResponse);
+            setResult(responseWithSymbol);
           }
           return;
         }