Add TypeScript typechecking

.github/workflows/typecheck.yml

@@ -0,0 +1,39 @@
+name: Typecheck
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+    paths:
+      - "**/*.[tj]s"
+      - "**/*.?[tj]s"
+      - "tsconfig.json"
+      - "package.json"
+      - "package-lock.json"
+      - ".nvmrc"
+      - ".github/workflows/typecheck.yml"
+
+jobs:
+  typecheck:
+    runs-on: blacksmith-2vcpu-ubuntu-2404
+    steps:
+      - name: Checkout
+        # 6.0.3
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10
+
+      # v6.1.0
+      - name: Setup Node
+        uses: actions/setup-node@395ad3262231945c25e8478fd5baf05154b1d79f
+        with:
+          node-version-file: .nvmrc
+          cache: npm
+          cache-dependency-path: package-lock.json
+
+      - name: Install npm packages
+        run: npm ci
+
+      - name: Typecheck
+        run: npm run typecheck
+
+      - name: Tell people how to run the failing check
+        if: failure()
+        run: echo "::error::The typecheck check failed! To run it locally, run \`npm ci && npm run typecheck\` from the GitHub-Actions repo root."

package.json

@@ -3,12 +3,15 @@
   "private": true,
   "type": "module",
   "scripts": {
+    "typecheck": "tsgo --noEmit -p tsconfig.json",
     "verify-peer-review": "tsx scripts/verifyPeerReview.ts"
   },
   "dependencies": {
     "expensify-common": "2.0.189"
   },
   "devDependencies": {
+    "@types/node": "^25.9.4",
+    "@typescript/native-preview": "7.0.0-dev.20260702.3",
     "tsx": "^4.22.5"
   }
 }

tsconfig.json

@@ -0,0 +1,13 @@
+{
+    "compilerOptions": {
+        "module": "ESNext",
+        "moduleResolution": "Bundler",
+        "target": "ES2022",
+        "types": ["node"],
+        "strict": true,
+        "noEmit": true,
+        "skipLibCheck": true,
+        "esModuleInterop": true
+    },
+    "include": ["scripts/**/*.ts", "tests/**/*.ts"]
+}