babel: Type-check Babel configuration.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -102,6 +102,8 @@
     "@formatjs/cli": "^6.0.0",
     "@giphy/js-types": "^5.1.0",
     "@types/autosize": "^4.0.1",
+    "@types/babel__core": "^7.20.5",
+    "@types/babel__preset-env": "^7.10.0",
     "@types/blueimp-md5": "^2.18.0",
     "@types/co-body": "^6.1.3",
     "@types/confusing-browser-globals": "^1.0.3",

pnpm-lock.yaml

@@ -327,6 +327,12 @@ importers:
       '@types/autosize':
         specifier: ^4.0.1
         version: 4.0.3
+      '@types/babel__core':
+        specifier: ^7.20.5
+        version: 7.20.5
+      '@types/babel__preset-env':
+        specifier: ^7.10.0
+        version: 7.10.0
       '@types/blueimp-md5':
         specifier: ^2.18.0
         version: 2.18.2
@@ -2673,6 +2679,9 @@ packages:
   '@types/babel__helper-plugin-utils@7.10.3':
     resolution: {integrity: sha512-FcLBBPXInqKfULB2nvOBskQPcnSMZ0s1Y2q76u9H1NPPWaLcTeq38xBeKfF/RBUECK333qeaqRdYoPSwW7rTNQ==}
 
+  '@types/babel__preset-env@7.10.0':
+    resolution: {integrity: sha512-LS8hRb/8TQir2f8W9/s5enDtrRS2F/6fsdkVw5ePHp6Q8SrSJHOGtWnP93ryaYMmg2du03vOsiGrl5mllz4uDA==}
+
   '@types/babel__template@7.4.4':
     resolution: {integrity: sha512-h/NUaSyG5EyxBIp8YRxo4RMe2/qQgvyowRwVMzhYhBCONbW8PUsg4lkFMrhgZhUe5z3L3MiLDuvyJ/CaPa2A8A==}
 
@@ -12312,6 +12321,8 @@ snapshots:
     dependencies:
       '@types/babel__core': 7.20.5
 
+  '@types/babel__preset-env@7.10.0': {}
+
   '@types/babel__template@7.4.4':
     dependencies:
       '@babel/parser': 7.28.3

version.py

@@ -49,4 +49,4 @@ API_FEATURE_LEVEL = 425
 #   historical commits sharing the same major version, in which case a
 #   minor version bump suffices.
 
-PROVISION_VERSION = (347, 3)  # bumped 2025-09-09 to add @types/eslint-config-prettier
+PROVISION_VERSION = (347, 4)  # bumped 2025-09-09 to add @types/babel__core, @types/babel__preset-env

web/babel.config.js

@@ -1,22 +1,20 @@
-export default {
-    plugins: [
-        [
-            "formatjs",
-            {
+// @ts-check
+
+/** @type {import("babel-plugin-formatjs/types").Options} */
+const formatJsOptions = {
     additionalFunctionNames: ["$t", "$t_html"],
-                overrideIdFn: (_id, defaultMessage) => defaultMessage,
-            },
-        ],
-    ],
-    presets: [
-        [
-            "@babel/preset-env",
-            {
+    overrideIdFn: (_id, defaultMessage) => defaultMessage ?? "",
+};
+
+/** @type {import("@babel/preset-env").Options} */
+const presetEnvOptions = {
     corejs: "3.45",
     shippedProposals: true,
     useBuiltIns: "usage",
-            },
-        ],
-        "@babel/typescript",
-    ],
+};
+
+/** @type {import("@babel/core").TransformOptions} */
+export default {
+    plugins: [["formatjs", formatJsOptions]],
+    presets: [["@babel/preset-env", presetEnvOptions], "@babel/typescript"],
 };