chore(null): null strictness fixes for /src/scanner/ files requiring …

…manual attention (#3341)

#### Description of changes

Cleanup for a few new strict-null-check cases

#### Pull request checklist
<!-- If a checklist item is not applicable to this change, write "n/a" in the checkbox -->
- [x] Addresses an existing issue: #2869
- [x] Ran `yarn fastpass`
- [x] Added/updated relevant unit test(s) (and ran `yarn test`)
- [x] Verified code coverage for the changes made. Check coverage report at: `<rootDir>/test-results/unit/coverage`
- [x] PR title *AND* final merge commit title both start with a semantic tag (`fix:`, `chore:`, `feat(feature-name):`, `refactor:`). See `CONTRIBUTING.md`.
- [n/a] (UI changes only) Added screenshots/GIFs to description above
- [n/a] (UI changes only) Verified usability with NVDA/JAWS

src/scanner/custom-rules/css-content-rule.ts

@@ -29,14 +29,14 @@ function pageHasElementsWithPseudoSelectors(node: HTMLElement): boolean {
     return pseudoElements.length > 0;
 }
 
-function getAllPseudoElements(node: HTMLElement): HTMLElement[] {
+function getAllPseudoElements(node: HTMLElement): Element[] {
     const elements = node.querySelectorAll('*');
 
     const hasContent = styles => {
         return styles && styles.content !== 'none';
     };
 
-    const pseudoElements = [];
+    const pseudoElements: Element[] = [];
     for (let index = 0; index < elements.length; index++) {
         const element = elements.item(index);
         const beforeStyles = window.getComputedStyle(element, ':before');

src/scanner/custom-rules/heading-rule.ts

@@ -21,8 +21,8 @@ export const headingConfiguration: RuleConfiguration = {
 
 function evaluateCodedHeadings(node: HTMLElement, options: any): boolean {
     const headingText: string = node.innerText;
-    let headingLevel: number;
-    const ariaHeadingLevel: string = node.getAttribute('aria-level');
+    let headingLevel: number | undefined;
+    const ariaHeadingLevel: string | null = node.getAttribute('aria-level');
     if (ariaHeadingLevel !== null) {
         headingLevel = parseInt(ariaHeadingLevel, 10);
     } else {

src/scanner/help-url-getter.ts

@@ -10,7 +10,7 @@ export class HelpUrlGetter {
         return customHelpUrl || axeHelpUrl;
     }
 
-    private getCustomHelpUrl(ruleId: string): string {
+    private getCustomHelpUrl(ruleId: string): string | null {
         for (let index = 0; index < this.ruleConfigs.length; index++) {
             const config = this.ruleConfigs[index];
             if (config.rule.id === ruleId && config.rule.helpUrl != null) {

src/scanner/processor.ts

@@ -12,7 +12,10 @@ export namespace Processor {
         // add messages to suppress here. Remove comment when non-empty.
     ].map(normalizeText);
 
-    export function suppressChecksByMessages(rule: AxeRule, removeEmptyRules = true): AxeRule {
+    export function suppressChecksByMessages(
+        rule: AxeRule,
+        removeEmptyRules = true,
+    ): AxeRule | null {
         rule.nodes = rule.nodes.filter((nodeResult: AxeNodeResult) => {
             nodeResult.any = nodeResult.any.filter((check: any) => {
                 const checkShown =

src/tests/unit/tests/scanner/custom-rules-configuration-stub.ts

@@ -8,7 +8,9 @@ const fakeRuleConfigurationA: RuleConfiguration = {
     checks: [
         {
             id: 'fake-check-id',
-            evaluate: () => null,
+            evaluate: () => {
+                throw 'unimplemented evaluate stub';
+            },
             passMessage: () => passMessageStub,
             failMessage: () => failMessageStub,
         },

tsconfig.strictNullChecks.json

@@ -302,16 +302,20 @@
         "./src/scanner/check-message-transformer.ts",
         "./src/scanner/custom-rules/autocomplete-rule.ts",
         "./src/scanner/custom-rules/color-rule.ts",
+        "./src/scanner/custom-rules/css-content-rule.ts",
         "./src/scanner/custom-rules/css-positioning-rule.ts",
         "./src/scanner/custom-rules/frame-title.ts",
         "./src/scanner/custom-rules/header-rule.ts",
+        "./src/scanner/custom-rules/heading-rule.ts",
         "./src/scanner/custom-rules/link-purpose.ts",
         "./src/scanner/custom-rules/page-title.ts",
         "./src/scanner/custom-rules/text-contrast.ts",
         "./src/scanner/custom-rules/unique-landmark.ts",
         "./src/scanner/document-utils.ts",
+        "./src/scanner/help-url-getter.ts",
         "./src/scanner/iruleresults.d.ts",
         "./src/scanner/locale-configuration.ts",
+        "./src/scanner/processor.ts",
         "./src/scanner/role-utils.ts",
         "./src/scanner/scan-options.ts",
         "./src/tests/common/get-automation-id-selector.ts",
@@ -357,6 +361,7 @@
         "./src/tests/unit/tests/electron/flux/action-creator/scan-result-example.ts",
         "./src/tests/unit/tests/electron/platform/android/setup/steps/actions-tester.ts",
         "./src/tests/unit/tests/injected/visualization/drawer-utils-mock-builder.ts",
+        "./src/tests/unit/tests/scanner/custom-rules-configuration-stub.ts",
         "./src/tests/unit/tests/scanner/mock-axe-utils.ts",
         "./src/views/content/guidance-title.tsx"
     ],