tests(smokehouse): assert on expected array length (#9292)

Co-Authored-By: Brendan Kenny <[email protected]>

lighthouse-cli/test/smokehouse/byte-efficiency/expectations.js

@@ -140,6 +140,9 @@ module.exports = [
                 transferSize: 53181,
                 resourceSize: 52997,
               },
+              {
+                url: 'http://localhost:10200/favicon.ico',
+              },
             ],
           },
         },

lighthouse-cli/test/smokehouse/error-expectations.js

@@ -5,6 +5,12 @@
  */
 'use strict';
 
+// Just using `[]` actually asserts for an empty array.
+// Use this expectation object to assert an array with at least one element.
+const NONEMPTY_ARRAY = {
+  length: '>0',
+};
+
 /**
  * Expected Lighthouse audit values for sites with various errors.
  */
@@ -24,10 +30,10 @@ module.exports = [
     artifacts: {
       PageLoadError: {code: 'PAGE_HUNG'},
       devtoolsLogs: {
-        'pageLoadError-defaultPass': [/* ... */],
+        'pageLoadError-defaultPass': NONEMPTY_ARRAY,
       },
       traces: {
-        'pageLoadError-defaultPass': {traceEvents: [/* ... */]},
+        'pageLoadError-defaultPass': {traceEvents: NONEMPTY_ARRAY},
       },
     },
   },
@@ -46,10 +52,10 @@ module.exports = [
     artifacts: {
       PageLoadError: {code: 'INSECURE_DOCUMENT_REQUEST'},
       devtoolsLogs: {
-        'pageLoadError-defaultPass': [/* ... */],
+        'pageLoadError-defaultPass': NONEMPTY_ARRAY,
       },
       traces: {
-        'pageLoadError-defaultPass': {traceEvents: [/* ... */]},
+        'pageLoadError-defaultPass': {traceEvents: NONEMPTY_ARRAY},
       },
     },
   },

lighthouse-cli/test/smokehouse/smokehouse-report.js

@@ -98,6 +98,18 @@ function findDifference(path, actual, expected) {
     }
   }
 
+  // If the expected value is an array, assert the length as well.
+  // This still allows for asserting that the first n elements of an array are specified elements,
+  // but requires using an object literal (ex: {0: x, 1: y, 2: z} matches [x, y, z, q, w, e] and
+  // {0: x, 1: y, 2: z, length: 5} does not match [x, y, z].
+  if (Array.isArray(expected) && actual.length !== expected.length) {
+    return {
+      path: `${path}.length`,
+      actual,
+      expected,
+    };
+  }
+
   return null;
 }