Log warning for bad cache control

packages/workbox-core/_default.mjs

@@ -19,6 +19,7 @@ import {WorkboxError} from './_private/WorkboxError.mjs';
 import {cacheNames} from './_private/cacheNames.mjs';
 import {logger} from './_private/logger.mjs';
 import {assert} from './_private/assert.mjs';
+import {checkSWFileCacheHeaders} from './_private/checkSWFileCacheHeaders.mjs';
 import {setLoggerLevel, getLoggerLevel} from './_private/logger.mjs';
 import './_version.mjs';
 
@@ -61,6 +62,10 @@ class WorkboxCore {
         `${padding}https://github.com/GoogleChrome/workbox/issues/new`
       );
       logger.groupEnd();
+
+      if (typeof checkSWFileCacheHeaders === 'function') {
+        checkSWFileCacheHeaders();
+      }
     }
   }
 

packages/workbox-core/_private/assert.mjs

@@ -96,7 +96,7 @@ const isArrayOfClass = (value, expectedClass,
   }
 };
 
-const finalExports = process.env.NODE_ENV === 'production' ? null : {
+const finalAssertExports = process.env.NODE_ENV === 'production' ? null : {
   hasMethod,
   isArray,
   isInstance,
@@ -106,4 +106,4 @@ const finalExports = process.env.NODE_ENV === 'production' ? null : {
   isArrayOfClass,
 };
 
-export {finalExports as assert};
+export {finalAssertExports as assert};

packages/workbox-core/_private/checkSWFileCacheHeaders.mjs

@@ -0,0 +1,86 @@
+/*
+  Copyright 2017 Google Inc.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+      https://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+import {logger} from './logger.mjs';
+import '../_version.mjs';
+
+const MAX_AGE_REGEX = /max-age\s*=\s*(\d*)/g;
+
+/**
+ * Logs a warning to the user recommending changing
+ * to max-age=0 or no-cache.
+ *
+ * @param {string} cacheControlHeader
+ *
+ * @private
+ */
+function showWarning(cacheControlHeader) {
+  const docsUrl = 'https://developers.google.com/web/tools/workbox/guides/service-worker-checklist#cache-control_of_your_service_worker_file';
+  logger.warn(`You are setting a 'cache-control' header of ` +
+    `'${cacheControlHeader}' on your service worker file. This should be ` +
+    `set to 'max-age=0' or 'no-cache' to ensure the latest service worker ` +
+    `is served to your users. Learn more here: ${docsUrl}`
+  );
+}
+
+/**
+ * Checks for cache-control header on SW file and
+ * warns the developer if it exists with a value
+ * other than max-age=0 or no-cache.
+ *
+ * @private
+ */
+function checkSWFileCacheHeaders() {
+  // This is wrapped as an iife to allow async/await while making
+  //  rollup exclude it in builds.
+  (async () => {
+    try {
+      const swFile = self.location.href;
+      const response = await fetch(swFile);
+      if (!response.ok) {
+        // Response failed so nothing we can check;
+        return;
+      }
+
+      if (!response.headers.has('cache-control')) {
+        // No cache control header.
+        return;
+      }
+
+      const cacheControlHeader = response.headers.get('cache-control');
+
+      const maxAgeResult = MAX_AGE_REGEX.exec(cacheControlHeader);
+      if (maxAgeResult) {
+        if (parseInt(maxAgeResult[1], 10) === 0) {
+          return;
+        }
+      }
+
+      if (cacheControlHeader.indexOf('no-cache') !== -1) {
+        return;
+      }
+
+      showWarning(cacheControlHeader);
+    } catch (err) {
+      // NOOP
+    }
+  })();
+}
+
+const finalCheckSWFileCacheHeaders =
+  process.env.NODE_ENV === 'production' ? null : checkSWFileCacheHeaders;
+
+export {finalCheckSWFileCacheHeaders as checkSWFileCacheHeaders};