Fix exception on missing manifest start_url

Author: paulirish

lighthouse-core/audits/cache-start-url.js

@@ -39,20 +39,23 @@ class CacheStartUrl extends Audit {
    */
   static audit(artifacts) {
     let cacheHasStartUrl = false;
+    const manifest = artifacts.Manifest && artifacts.Manifest.value;
+    const cacheContents = artifacts.CacheContents;
+    const baseURL = artifacts.URL;
 
-    if (!(artifacts.Manifest &&
-          artifacts.Manifest.value &&
-          artifacts.Manifest.value.start_url &&
-          Array.isArray(artifacts.CacheContents) &&
-          artifacts.URL)) {
+    if (!(manifest && manifest.start_url && manifest.start_url.raw)) {
       return CacheStartUrl.generateAuditResult({
-        rawValue: false
+        rawValue: false,
+        debugString: 'start_url not present in Manifest'
       });
     }
 
-    const manifest = artifacts.Manifest.value;
-    const cacheContents = artifacts.CacheContents;
-    const baseURL = artifacts.URL;
+    if (!(Array.isArray(cacheContents) && artifacts.URL)) {
+      return CacheStartUrl.generateAuditResult({
+        rawValue: false,
+        debugString: 'No cache or URL detected'
+      });
+    }
 
     // Remove any UTM strings.
     const startURL = url.resolve(baseURL, manifest.start_url.raw).toString();

lighthouse-core/test/audits/cache-start-url.js

@@ -43,6 +43,18 @@ describe('Cache: start_url audit', () => {
     return assert.equal(Audit.audit({Manifest, CacheContents}).rawValue, false);
   });
 
+  // Need to disable camelcase check for dealing with short_name.
+  /* eslint-disable camelcase */
+  it('fails when a manifest contains no start_url', () => {
+    const inputs = {
+      Manifest: {
+
+      }
+    };
+
+    return assert.equal(Audit.audit(inputs).rawValue, false);
+  });
+
   // Need to disable camelcase check for dealing with short_name.
   /* eslint-disable camelcase */
   it('fails when a manifest contains no start_url', () => {