NathanWalker · Oct 7, 2026
diff --git a/.github/workflows/core_typecheck.yml b/.github/workflows/core_typecheck.yml
@@ -0,0 +1,33 @@
+# Type-checks packages/core under strictNullChecks against the per-file error
+# baseline in packages/core/strict-baseline.json, so the error count can only
+# go down.
+name: 'Core strict type-check'
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+
+env:
+  NX_CLOUD_ACCESS_TOKEN: ${{ secrets.NX_CLOUD_ACCESS_TOKEN }}
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  typecheck-strict:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
+        with:
+          node-version: 23.5.0
+          cache: 'npm'
+      - name: 'Install dependencies'
+        run: npm ci
+      - name: 'Check strictNullChecks baseline'
+        run: npx nx run core:typecheck-strict
diff --git a/AGENTS.md b/AGENTS.md
@@ -57,6 +57,7 @@ To add a skill: create `.agent/skills/<kebab-case-name>/SKILL.md` with `name` an
   - Watch mode: `npx nx run core:test --watch`
   - Single suite by describe name: `npx nx run core:test -t 'XmlParser'`
 - Unit tests run in Node with NativeScript platform globals mocked in `packages/core/vitest.setup.ts` — they cannot exercise real native APIs. Behavior that touches iOS/Android at runtime is covered by the e2e suite: `npx nx run apps-automated:ios` or `npx nx run apps-automated:android` (requires a configured NativeScript environment with simulators/emulators).
+- `npx nx run core:typecheck-strict` type-checks core under `strictNullChecks` against the per-file error counts in `packages/core/strict-baseline.json` (CI enforces it). Fixing strict errors? Run it with `--update` to lower the baseline in the same change; it refuses to record increases.
 - Prefer adding a unit test for logic changes; add or extend an `apps/automated` test for native runtime behavior.
 
 ## Formatting & Commits

diff --git a/packages/core/project.json b/packages/core/project.json
@@ -63,6 +63,12 @@
 			"options": {
 				"reportsDirectory": "../../coverage/packages/core"
 			}
+		},
+		"typecheck-strict": {
+			"executor": "nx:run-commands",
+			"options": {
+				"command": "node tools/scripts/strict-baseline.mjs"
+			}
 		}
 	}
 }
diff --git a/packages/core/strict-baseline.json b/packages/core/strict-baseline.json
@@ -0,0 +1,178 @@
+{
+	"accessibility/accessibility-common.ts": 4,
+	"accessibility/accessibility-properties.ts": 3,
+	"application-settings/index.android.ts": 9,
+	"application-settings/index.ios.ts": 9,
+	"application/application-common.ts": 5,
+	"application/application.android.ts": 21,
+	"application/application.ios.ts": 33,
+	"application/helpers.android.ts": 2,
+	"color/color-common.ts": 2,
+	"color/color-utils.ts": 1,
+	"color/known-colors.ts": 1,
+	"connectivity/index.android.ts": 2,
+	"connectivity/index.ios.ts": 3,
+	"css-mediaquery/index.ts": 5,
+	"css/CSS3Parser.ts": 28,
+	"css/CSSNativeScript.ts": 5,
+	"css/css-tree-parser.ts": 2,
+	"css/lib/parse/index.ts": 4,
+	"css/parser.ts": 32,
+	"css/system-classes.ts": 3,
+	"data/observable-array/index.ts": 9,
+	"data/observable/index.ts": 2,
+	"data/virtual-array/index.ts": 1,
+	"debugger/dom-types.ts": 2,
+	"debugger/webinspector-css.ts": 1,
+	"debugger/webinspector-dom.ts": 3,
+	"debugger/webinspector-network.android.ts": 3,
+	"debugger/webinspector-network.ios.ts": 2,
+	"file-system/file-system-access.android.ts": 42,
+	"file-system/file-system-access.ios.ts": 11,
+	"file-system/index.ts": 12,
+	"globals/global-utils.ts": 2,
+	"globals/index.ts": 3,
+	"http/http-request-internal/index.android.ts": 8,
+	"http/http-request-internal/index.ios.ts": 3,
+	"http/http-request/index.android.ts": 2,
+	"http/http-request/index.ios.ts": 1,
+	"http/index.ts": 5,
+	"image-asset/index.ios.ts": 1,
+	"image-source/index.android.ts": 15,
+	"image-source/index.ios.ts": 31,
+	"inspector_modules.ts": 2,
+	"media-query-list/index.ts": 1,
+	"module-name-resolver/helpers.ts": 1,
+	"module-name-resolver/index.ts": 1,
+	"module-name-resolver/qualifier-matcher/index.ts": 6,
+	"native-window/native-window-common.ts": 6,
+	"native-window/native-window.android.ts": 3,
+	"native-window/native-window.ios.ts": 2,
+	"profiling/index.ts": 3,
+	"timer/index.android.ts": 1,
+	"timer/index.ios.ts": 1,
+	"trace/index.ts": 2,
+	"ui/action-bar/action-bar-common.ts": 5,
+	"ui/action-bar/index.android.ts": 10,
+	"ui/action-bar/index.ios.ts": 32,
+	"ui/animation/animation-common.ts": 6,
+	"ui/animation/index.android.ts": 3,
+	"ui/animation/index.ios.ts": 16,
+	"ui/animation/keyframe-animation.ts": 15,
+	"ui/builder/binding-builder.ts": 12,
+	"ui/builder/index.ts": 10,
+	"ui/button/index.ios.ts": 17,
+	"ui/content-view/index.ts": 1,
+	"ui/core/bindable/bindable-expressions.ts": 6,
+	"ui/core/bindable/index.ts": 21,
+	"ui/core/properties/index.ts": 21,
+	"ui/core/view-base/index.ts": 21,
+	"ui/core/view/index.android.ts": 26,
+	"ui/core/view/index.ios.ts": 31,
+	"ui/core/view/view-common.ts": 24,
+	"ui/core/view/view-helper/index.android.ts": 1,
+	"ui/core/view/view-helper/index.ios.ts": 28,
+	"ui/core/view/view-helper/view-helper-common.ts": 5,
+	"ui/core/weak-event-listener/index.ts": 1,
+	"ui/date-picker/index.ios.ts": 4,
+	"ui/dialogs/dialogs-common.ts": 5,
+	"ui/dialogs/index.android.ts": 13,
+	"ui/dialogs/index.ios.ts": 14,
+	"ui/editable-text-base/index.android.ts": 12,
+	"ui/editable-text-base/index.ios.ts": 3,
+	"ui/frame/fragment.android.ts": 1,
+	"ui/frame/fragment.transitions.android.ts": 60,
+	"ui/frame/frame-common.ts": 21,
+	"ui/frame/frame-helper-for-android.ts": 14,
+	"ui/frame/index.android.ts": 19,
+	"ui/frame/index.ios.ts": 28,
+	"ui/gestures/gestures-common.ts": 2,
+	"ui/gestures/index.android.ts": 17,
+	"ui/gestures/index.ios.ts": 23,
+	"ui/gestures/touch-manager.ts": 4,
+	"ui/html-view/index.ios.ts": 3,
+	"ui/image-cache/image-cache-common.ts": 5,
+	"ui/image-cache/index.ios.ts": 1,
+	"ui/image/image-common.ts": 5,
+	"ui/image/index.android.ts": 5,
+	"ui/image/index.ios.ts": 2,
+	"ui/label/index.ios.ts": 1,
+	"ui/layouts/flexbox-layout/flexbox-layout-common.ts": 1,
+	"ui/layouts/flexbox-layout/index.ios.ts": 1,
+	"ui/layouts/grid-layout/index.android.ts": 2,
+	"ui/layouts/layout-base-common.ts": 2,
+	"ui/layouts/root-layout/index.android.ts": 24,
+	"ui/layouts/root-layout/index.ios.ts": 10,
+	"ui/layouts/root-layout/root-layout-common.ts": 27,
+	"ui/layouts/root-layout/root-layout-stack.ts": 2,
+	"ui/list-picker/index.android.ts": 4,
+	"ui/list-picker/index.ios.ts": 3,
+	"ui/list-picker/list-picker-common.ts": 1,
+	"ui/list-view/index.android.ts": 23,
+	"ui/list-view/index.ios.ts": 17,
+	"ui/list-view/list-view-common.ts": 3,
+	"ui/page/index.android.ts": 1,
+	"ui/page/index.ios.ts": 24,
+	"ui/progress/index.android.ts": 3,
+	"ui/progress/index.ios.ts": 3,
+	"ui/proxy-view-container/index.ts": 3,
+	"ui/repeater/index.ts": 6,
+	"ui/scroll-view/index.android.ts": 1,
+	"ui/scroll-view/index.ios.ts": 1,
+	"ui/search-bar/index.android.ts": 5,
+	"ui/search-bar/index.ios.ts": 9,
+	"ui/segmented-bar/index.android.ts": 3,
+	"ui/segmented-bar/index.ios.ts": 6,
+	"ui/slider/index.android.ts": 2,
+	"ui/slider/index.ios.ts": 6,
+	"ui/split-view/index.ios.ts": 3,
+	"ui/styling/background-common.ts": 1,
+	"ui/styling/background.android.ts": 11,
+	"ui/styling/background.ios.ts": 44,
+	"ui/styling/converters.ts": 2,
+	"ui/styling/css-animation-parser.ts": 6,
+	"ui/styling/css-selector.ts": 25,
+	"ui/styling/css-shadow.ts": 2,
+	"ui/styling/css-stroke.ts": 2,
+	"ui/styling/css-transform.ts": 1,
+	"ui/styling/css-utils.ts": 1,
+	"ui/styling/font-common.ts": 4,
+	"ui/styling/font.android.ts": 10,
+	"ui/styling/font.ios.ts": 7,
+	"ui/styling/linear-gradient.ts": 1,
+	"ui/styling/style-properties.ts": 10,
+	"ui/styling/style-scope.ts": 38,
+	"ui/styling/style/index.ts": 3,
+	"ui/switch/index.ios.ts": 5,
+	"ui/tab-view/index.android.ts": 21,
+	"ui/tab-view/index.ios.ts": 27,
+	"ui/tab-view/tab-view-common.ts": 1,
+	"ui/text-base/index.android.ts": 14,
+	"ui/text-base/index.ios.ts": 22,
+	"ui/text-base/text-base-common.ts": 1,
+	"ui/text-field/index.ios.ts": 9,
+	"ui/text-view/index.ios.ts": 6,
+	"ui/time-picker/index.ios.ts": 2,
+	"ui/time-picker/time-picker-common.ts": 4,
+	"ui/transition/fade-transition.android.ts": 4,
+	"ui/transition/flip-transition.android.ts": 4,
+	"ui/transition/modal-transition.ios.ts": 12,
+	"ui/transition/page-transition.android.ts": 15,
+	"ui/transition/page-transition.ios.ts": 6,
+	"ui/transition/shared-transition-helper.ios.ts": 203,
+	"ui/transition/slide-transition.android.ts": 16,
+	"ui/transition/slide-transition.ios.ts": 1,
+	"ui/web-view/index.ios.ts": 9,
+	"ui/web-view/web-view-common.ts": 1,
+	"utils/common.ts": 5,
+	"utils/debug-source.ts": 2,
+	"utils/index.android.ts": 3,
+	"utils/index.ios.ts": 3,
+	"utils/native-helper-for-android.ts": 17,
+	"utils/native-helper.ios.ts": 11,
+	"utils/shared.ts": 1,
+	"utils/types.ts": 3,
+	"wgc/crypto/SubtleCrypto.ts": 46,
+	"xhr/index.ts": 10,
+	"xml/index.ts": 13
+}
diff --git a/packages/core/tsconfig.strict.json b/packages/core/tsconfig.strict.json
@@ -0,0 +1,8 @@
+{
+	"extends": "./tsconfig.lib.json",
+	"compilerOptions": {
+		"strictNullChecks": true,
+		"noEmit": true,
+		"diagnostics": false
+	}
+}
diff --git a/tools/notes/DevelopmentWorkflow.md b/tools/notes/DevelopmentWorkflow.md
@@ -54,6 +54,15 @@ Run a single test by it's describe name, for example to run just the `xml/index.
 npx nx run core:test --watch -t 'XmlParser' 
 ```
 
+## Strict null checks
+
+`packages/core` is not yet clean under `strictNullChecks`. CI type-checks it in strict mode and compares the error count of each file with `packages/core/strict-baseline.json`: a file may not gain errors, and a file that loses some must lower its baseline in the same change.
+
+```bash
+npx nx run core:typecheck-strict           # check against the baseline
+npx nx run core:typecheck-strict --update  # record lowered counts
+```
+
 ## Running the `e2e` Test Apps
 
 There are a couple of application used for development and testing.

diff --git a/tools/scripts/strict-baseline.mjs b/tools/scripts/strict-baseline.mjs
@@ -0,0 +1,87 @@
+// Ratchet for type-checking packages/core under strictNullChecks: per-file error
+// counts must match packages/core/strict-baseline.json. Counts are kept per file
+// so an error fixed in one file cannot hide one added in another.
+//
+//   node tools/scripts/strict-baseline.mjs           check against the baseline
+//   node tools/scripts/strict-baseline.mjs --update  record lowered counts
+import { spawnSync } from 'node:child_process';
+import { existsSync, readFileSync, writeFileSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import { dirname, join, relative } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const root = join(dirname(fileURLToPath(import.meta.url)), '../..');
+const coreDir = join(root, 'packages/core');
+const baselinePath = join(coreDir, 'strict-baseline.json');
+const baselineName = relative(root, baselinePath);
+const update = process.argv.includes('--update');
+
+const tsc = createRequire(import.meta.url).resolve('typescript/bin/tsc');
+const run = spawnSync(process.execPath, [tsc, '-p', 'tsconfig.strict.json', '--pretty', 'false'], { cwd: coreDir, encoding: 'utf8', maxBuffer: 256 * 1024 * 1024 });
+if (run.error) throw run.error;
+
+// A diagnostic starts at column 0; its continuation lines are indented.
+const FILE_ERROR = /^(.+)\(\d+,\d+\): error TS\d+:/;
+const diagnostics = [];
+for (const line of run.stdout.split('\n')) {
+	if (/^\S/.test(line)) diagnostics.push(line);
+	else if (line && diagnostics.length) diagnostics[diagnostics.length - 1] += '\n' + line;
+}
+
+const current = {};
+const errorsByFile = {};
+for (const diagnostic of diagnostics) {
+	const match = FILE_ERROR.exec(diagnostic);
+	if (!match) {
+		// Errors without a file (bad config, missing lib) cannot be baselined.
+		console.error(run.stdout + run.stderr);
+		fail('tsc reported an error that is not tied to a file.');
+	}
+	const file = match[1];
+	current[file] = (current[file] ?? 0) + 1;
+	(errorsByFile[file] ??= []).push(diagnostic);
+}
+if (run.status !== 0 && diagnostics.length === 0) {
+	console.error(run.stdout + run.stderr);
+	fail(`tsc exited with ${run.status} without reporting errors.`);
+}
+
+const hasBaseline = existsSync(baselinePath);
+if (!hasBaseline && !update) fail(`${baselineName} is missing; create it with --update.`);
+const baseline = hasBaseline ? JSON.parse(readFileSync(baselinePath, 'utf8')) : {};
+const files = [...new Set([...Object.keys(baseline), ...Object.keys(current)])].sort();
+const increased = files.filter((file) => (current[file] ?? 0) > (baseline[file] ?? 0));
+const decreased = files.filter((file) => (current[file] ?? 0) < (baseline[file] ?? 0));
+const total = (counts) => Object.values(counts).reduce((sum, count) => sum + count, 0);
+const describe = (file) => `  ${file}: ${baseline[file] ?? 0} -> ${current[file] ?? 0}`;
+
+if (hasBaseline && increased.length) {
+	for (const file of increased) console.error(errorsByFile[file].join('\n'));
+	console.error(`\nstrictNullChecks errors increased in ${increased.length} file(s):`);
+	console.error(increased.map(describe).join('\n'));
+	fail(`Fix the new errors above; ${baselineName} only goes down${update ? ', so it was not updated' : ''}.`);
+}
+
+if (update) {
+	const next = Object.fromEntries(
+		Object.keys(current)
+			.sort()
+			.map((file) => [file, current[file]]),
+	);
+	writeFileSync(baselinePath, JSON.stringify(next, null, '\t') + '\n');
+	console.log(`Wrote ${baselineName}: ${total(next)} errors in ${Object.keys(next).length} files (was ${total(baseline)}).`);
+	process.exit(0);
+}
+
+if (decreased.length) {
+	console.error(`strictNullChecks errors decreased in ${decreased.length} file(s):`);
+	console.error(decreased.map(describe).join('\n'));
+	fail(`Lower the baseline in the same change: npx nx run core:typecheck-strict --update`);
+}
+
+console.log(`strictNullChecks: ${total(current)} errors in ${Object.keys(current).length} files, matching ${baselineName}.`);
+
+function fail(message) {
+	console.error(message);
+	process.exit(1);
+}