fix: tweak fitness function rule for mocha tests (#20887)

development/fitness-functions/common/shared.ts

@@ -32,6 +32,14 @@ function filterDiffByFilePath(diff: string, regex: string): string {
   return filteredDiff;
 }
 
+// This function returns all lines that are additions to files that are being
+// modified but that previously already existed. Example:
+// diff --git a/test.js b/test.js
+// index 0000000000..872e0d8293
+// --- /dev/null
+// +++ b/test.js
+// @@ -0,0 +1 @@
+// +new line change to a previously existing file
 function filterDiffLineAdditions(diff: string): string {
   const diffLines = diff.split('\n');
 
@@ -44,6 +52,15 @@ function filterDiffLineAdditions(diff: string): string {
   return diffAdditionLines.join('/n');
 }
 
+// This function returns all lines that are additions to new files that are being
+// created. Example:
+// diff --git a/test.js b/test.js
+// new file mode 100644
+// index 0000000000..872e0d8293
+// --- /dev/null
+// +++ b/test.js
+// @@ -0,0 +1 @@
+// +new line change as the new file is created
 function filterDiffFileCreations(diff: string): string {
   // split by `diff --git` and remove the first element which is empty
   const diffBlocks = diff.split(`diff --git`).slice(1);
@@ -91,7 +108,7 @@ function hasNumberOfCodeBlocksIncreased(
 
 export {
   filterDiffByFilePath,
-  filterDiffLineAdditions,
   filterDiffFileCreations,
+  filterDiffLineAdditions,
   hasNumberOfCodeBlocksIncreased,
 };

development/fitness-functions/rules/sinon-assert-syntax.test.ts

@@ -1,4 +1,7 @@
-import { generateModifyFilesDiff } from '../common/test-data';
+import {
+  generateCreateFileDiff,
+  generateModifyFilesDiff,
+} from '../common/test-data';
 import { preventSinonAssertSyntax } from './sinon-assert-syntax';
 
 describe('preventSinonAssertSyntax()', (): void => {
@@ -10,7 +13,7 @@ describe('preventSinonAssertSyntax()', (): void => {
     expect(hasRulePassed).toBe(true);
   });
 
-  it('should not pass when receiving a diff with one of the blocked expressions', (): void => {
+  it('should pass when receiving a diff with an existing file with one of the blocked expressions', (): void => {
     const infringingExpression = 'assert.equal';
     const testDiff = [
       generateModifyFilesDiff('new-file.ts', 'foo', 'bar'),
@@ -24,6 +27,22 @@ describe('preventSinonAssertSyntax()', (): void => {
 
     const hasRulePassed = preventSinonAssertSyntax(testDiff);
 
+    expect(hasRulePassed).toBe(true);
+  });
+
+  it('should not pass when receiving a diff with a new file with one of the blocked expressions', (): void => {
+    const infringingExpression = 'assert.equal';
+    const testDiff = [
+      generateModifyFilesDiff('new-file.ts', 'foo', 'bar'),
+      generateCreateFileDiff('old-file.js', 'pong'),
+      generateCreateFileDiff(
+        'test.js',
+        `yada yada ${infringingExpression} yada yada`,
+      ),
+    ].join('');
+
+    const hasRulePassed = preventSinonAssertSyntax(testDiff);
+
     expect(hasRulePassed).toBe(false);
   });
 });

development/fitness-functions/rules/sinon-assert-syntax.ts

@@ -1,7 +1,7 @@
 import { EXCLUDE_E2E_TESTS_REGEX } from '../common/constants';
 import {
-  filterDiffLineAdditions,
   filterDiffByFilePath,
+  filterDiffFileCreations,
   hasNumberOfCodeBlocksIncreased,
 } from '../common/shared';
 
@@ -16,7 +16,7 @@ const codeBlocks = [
 
 function preventSinonAssertSyntax(diff: string): boolean {
   const diffByFilePath = filterDiffByFilePath(diff, EXCLUDE_E2E_TESTS_REGEX);
-  const diffAdditions = filterDiffLineAdditions(diffByFilePath);
+  const diffAdditions = filterDiffFileCreations(diffByFilePath);
   const hashmap = hasNumberOfCodeBlocksIncreased(diffAdditions, codeBlocks);
 
   const haveOccurencesOfAtLeastOneCodeBlockIncreased =