Add GitHub Actions tests to verify package exports

.github/workflows/tests.yml

@@ -204,6 +204,58 @@ jobs:
       - name: Run verify task
         run: ${{ matrix.task.run }}
 
+  package:
+    name: Export ${{ matrix.conditions }}, Node.js ${{ matrix.node-version }}
+    runs-on: ${{ github.event.inputs.runner || 'ubuntu-latest' }}
+    needs: [install, build]
+
+    # Skip when scheduled or run manually
+    if: ${{ !inputs.runner }}
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        node-version:
+          - 12.18 # Node.js 12.18 uses package exports with trailing slashes
+          - 12 # But Node.js 12.20+ uses package exports with subpath patterns
+          - 18
+
+        conditions:
+          - require
+          - import
+
+        exclude:
+          - conditions: import
+            node-version: 12.18
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Restore dependencies
+        uses: ./.github/workflows/actions/install-node
+
+      - name: Restore build
+        uses: ./.github/workflows/actions/build
+
+      - name: Change Node.js version
+        uses: actions/setup-node@v3
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - name: Export ${{ matrix.conditions }}, Node.js ${{ matrix.node-version }}
+        env:
+          # Node.js conditions override from "require" to "import" etc
+          # https://nodejs.org/api/cli.html#-c-condition---conditionscondition
+          FLAGS: ${{ matrix.conditions != 'require' && format(' --conditions {0}', matrix.conditions) || '' }}
+
+        run: |
+          node --eval "console.log(require.resolve('govuk-frontend'))"${{ env.FLAGS }}
+          node --eval "console.log(require.resolve('govuk-frontend/package.json'))"${{ env.FLAGS }}
+          node --eval "console.log(require.resolve('govuk-frontend/dist/govuk/i18n.js'))"${{ env.FLAGS }}
+          node --eval "console.log(require.resolve('govuk-frontend/dist/govuk-esm/i18n.mjs'))"${{ env.FLAGS }}
+
   regression:
     name: Percy
     needs: [install, build]