feat: Support .cts as configuration file

babel.config.js

@@ -464,17 +464,30 @@ function pluginPolyfillsOldNode({ template, types: t }) {
     },
   };
 }
+
+/**
+ * @param {import("@babel/core")} pluginAPI
+ * @returns {import("@babel/core").PluginObj}
+ */
 function pluginToggleBooleanFlag({ types: t }, { name, value }) {
+  function check(test) {
+    let keepConsequent = value;
+
+    if (test.isUnaryExpression({ operator: "!" })) {
+      test = test.get("argument");
+      keepConsequent = !keepConsequent;
+    }
+    return {
+      test,
+      keepConsequent,
+    };
+  }
+
   return {
     visitor: {
       "IfStatement|ConditionalExpression"(path) {
-        let test = path.get("test");
-        let keepConsequent = value;
-
-        if (test.isUnaryExpression({ operator: "!" })) {
-          test = test.get("argument");
-          keepConsequent = !keepConsequent;
-        }
+        // eslint-disable-next-line prefer-const
+        let { test, keepConsequent } = check(path.get("test"));
 
         // yarn-plugin-conditions injects bool(process.env.BABEL_8_BREAKING)
         // tests, to properly cast the env variable to a boolean.
@@ -494,6 +507,26 @@ function pluginToggleBooleanFlag({ types: t }, { name, value }) {
             : path.node.alternate || t.emptyStatement()
         );
       },
+      LogicalExpression(path) {
+        const { test, keepConsequent } = check(path.get("left"));
+
+        if (!test.matchesPattern(name)) return;
+
+        switch (path.node.operator) {
+          case "&&":
+            path.replaceWith(
+              keepConsequent ? path.node.right : t.booleanLiteral(false)
+            );
+            break;
+          case "||":
+            path.replaceWith(
+              keepConsequent ? t.booleanLiteral(true) : path.node.right
+            );
+            break;
+          default:
+            throw path.buildCodeFrameError("This check could not be stripped.");
+        }
+      },
       MemberExpression(path) {
         if (path.matchesPattern(name)) {
           throw path.buildCodeFrameError("This check could not be stripped.");

packages/babel-core/package.json

@@ -74,7 +74,8 @@
     "@types/gensync": "^1.0.0",
     "@types/resolve": "^1.3.2",
     "@types/semver": "^5.4.0",
-    "rimraf": "^3.0.0"
+    "rimraf": "^3.0.0",
+    "ts-node": "^10.9.1"
   },
   "conditions": {
     "BABEL_8_BREAKING": [

packages/babel-core/src/config/files/configuration.ts

@@ -9,7 +9,7 @@ import type { CacheConfigurator } from "../caching";
 import { makeConfigAPI } from "../helpers/config-api";
 import type { ConfigAPI } from "../helpers/config-api";
 import { makeStaticFileCache } from "./utils";
-import loadCjsOrMjsDefault from "./module-types";
+import loadCodeDefault from "./module-types";
 import pathPatternToRegex from "../pattern-to-regex";
 import type { FilePackageData, RelativeConfig, ConfigFile } from "./types";
 import type { CallerMetadata } from "../validation/options";
@@ -28,20 +28,22 @@ export const ROOT_CONFIG_FILENAMES = [
   "babel.config.cjs",
   "babel.config.mjs",
   "babel.config.json",
+  "babel.config.cts",
 ];
 const RELATIVE_CONFIG_FILENAMES = [
   ".babelrc",
   ".babelrc.js",
   ".babelrc.cjs",
   ".babelrc.mjs",
   ".babelrc.json",
+  ".babelrc.cts",
 ];
 
 const BABELIGNORE_FILENAME = ".babelignore";
 
 const LOADING_CONFIGS = new Set();
 
-const readConfigJS = makeStrongCache(function* readConfigJS(
+const readConfigCode = makeStrongCache(function* readConfigCode(
   filepath: string,
   cache: CacheConfigurator<{
     envName: string;
@@ -70,7 +72,7 @@ const readConfigJS = makeStrongCache(function* readConfigJS(
   let options: unknown;
   try {
     LOADING_CONFIGS.add(filepath);
-    options = yield* loadCjsOrMjsDefault(
+    options = yield* loadCodeDefault(
       filepath,
       "You appear to be using a native ECMAScript module configuration " +
         "file, which is only supported when running Babel asynchronously.",
@@ -313,9 +315,15 @@ function readConfig(
   caller: CallerMetadata | undefined,
 ): Handler<ConfigFile | null> {
   const ext = path.extname(filepath);
-  return ext === ".js" || ext === ".cjs" || ext === ".mjs"
-    ? readConfigJS(filepath, { envName, caller })
-    : readConfigJSON5(filepath);
+  switch (ext) {
+    case ".js":
+    case ".cjs":
+    case ".mjs":
+    case ".cts":
+      return readConfigCode(filepath, { envName, caller });
+    default:
+      return readConfigJSON5(filepath);
+  }
 }
 
 export function* resolveShowConfigPath(

packages/babel-core/src/config/files/module-types.ts

@@ -8,6 +8,9 @@ import semver from "semver";
 import { endHiddenCallStack } from "../../errors/rewrite-stack-trace";
 import ConfigError from "../../errors/config-error";
 
+import type { InputOptions } from "..";
+import { transformFileSync } from "../../transform-file";
+
 const require = createRequire(import.meta.url);
 
 let import_: ((specifier: string | URL) => any) | undefined;
@@ -23,38 +26,87 @@ export const supportsESM = semver.satisfies(
   "^12.17 || >=13.2",
 );
 
-export default function* loadCjsOrMjsDefault(
+export default function* loadCodeDefault(
   filepath: string,
   asyncError: string,
   // TODO(Babel 8): Remove this
   fallbackToTranspiledModule: boolean = false,
 ): Handler<unknown> {
-  switch (guessJSModuleType(filepath)) {
-    case "cjs":
+  switch (path.extname(filepath)) {
+    case ".cjs":
       return loadCjsDefault(filepath, fallbackToTranspiledModule);
-    case "unknown":
+    case ".mjs":
+      break;
+    case ".cts":
+      return loadCtsDefault(filepath);
+    default:
       try {
         return loadCjsDefault(filepath, fallbackToTranspiledModule);
       } catch (e) {
         if (e.code !== "ERR_REQUIRE_ESM") throw e;
       }
-    // fall through
-    case "mjs":
-      if (yield* isAsync()) {
-        return yield* waitFor(loadMjsDefault(filepath));
-      }
-      throw new ConfigError(asyncError, filepath);
   }
+  if (yield* isAsync()) {
+    return yield* waitFor(loadMjsDefault(filepath));
+  }
+  throw new ConfigError(asyncError, filepath);
 }
 
-function guessJSModuleType(filename: string): "cjs" | "mjs" | "unknown" {
-  switch (path.extname(filename)) {
-    case ".cjs":
-      return "cjs";
-    case ".mjs":
-      return "mjs";
-    default:
-      return "unknown";
+function loadCtsDefault(filepath: string) {
+  const ext = ".cts";
+  const hasTsSupport = !!(
+    require.extensions[".ts"] ||
+    require.extensions[".cts"] ||
+    require.extensions[".mts"]
+  );
+
+  let handler: NodeJS.RequireExtensions[""];
+
+  if (!hasTsSupport) {
+    const opts: InputOptions = {
+      babelrc: false,
+      configFile: false,
+      sourceType: "script",
+      sourceMaps: "inline",
+      presets: [
+        [
+          getTSPreset(filepath),
+          {
+            disallowAmbiguousJSXLike: true,
+            allExtensions: true,
+            onlyRemoveTypeImports: true,
+            optimizeConstEnums: true,
+            ...(!process.env.BABEL_8_BREAKING && {
+              allowDeclareFields: true,
+            }),
+          },
+        ],
+      ],
+    };
+
+    handler = function (m, filename) {
+      // If we want to support `.ts`, `.d.ts` must be handled specially.
+      if (handler && filename.endsWith(ext)) {
+        // @ts-expect-error Undocumented API
+        return m._compile(
+          transformFileSync(filename, {
+            ...opts,
+            filename,
+          }).code,
+          filename,
+        );
+      }
+      return require.extensions[".js"](m, filename);
+    };
+    require.extensions[ext] = handler;
+  }
+  try {
+    return endHiddenCallStack(require)(filepath);
+  } finally {
+    if (!hasTsSupport) {
+      if (require.extensions[ext] === handler) delete require.extensions[ext];
+      handler = undefined;
+    }
   }
 }
 
@@ -69,8 +121,7 @@ function loadCjsDefault(filepath: string, fallbackToTranspiledModule: boolean) {
 async function loadMjsDefault(filepath: string) {
   if (!import_) {
     throw new ConfigError(
-      "Internal error: Native ECMAScript modules aren't supported" +
-        " by this platform.\n",
+      "Internal error: Native ECMAScript modules aren't supported by this platform.\n",
       filepath,
     );
   }
@@ -80,3 +131,32 @@ async function loadMjsDefault(filepath: string) {
   const module = await endHiddenCallStack(import_)(pathToFileURL(filepath));
   return module.default;
 }
+
+function getTSPreset(filepath: string) {
+  try {
+    // eslint-disable-next-line import/no-extraneous-dependencies
+    return require("@babel/preset-typescript");
+  } catch (error) {
+    if (error.code !== "MODULE_NOT_FOUND") throw error;
+
+    let message =
+      "You appear to be using a .cts file as Babel configuration, but the `@babel/preset-typescript` package was not found: please install it!";
+
+    if (process.versions.pnp) {
+      // Using Yarn PnP, which doesn't allow requiring packages that are not
+      // explicitly specified as dependencies.
+      // TODO(Babel 8): Explicitly add `@babel/preset-typescript` as an
+      // optional peer dependency of `@babel/core`.
+      message += `
+If you are using Yarn Plug'n'Play, you may also need to add the following configuration to your .yarnrc.yml file:
+
+packageExtensions:
+\t"@babel/core@*":
+\t\tpeerDependencies:
+\t\t\t"@babel/preset-typescript": "*"
+`;
+    }
+
+    throw new ConfigError(message, filepath);
+  }
+}

packages/babel-core/src/config/files/plugins.ts

@@ -6,7 +6,7 @@ import buildDebug from "debug";
 import path from "path";
 import gensync, { type Handler } from "gensync";
 import { isAsync } from "../../gensync-utils/async";
-import loadCjsOrMjsDefault, { supportsESM } from "./module-types";
+import loadCodeDefault, { supportsESM } from "./module-types";
 import { fileURLToPath, pathToFileURL } from "url";
 
 import importMetaResolve from "./import-meta-resolve";
@@ -217,7 +217,7 @@ function* requireModule(type: string, name: string): Handler<unknown> {
     if (!process.env.BABEL_8_BREAKING) {
       LOADING_MODULES.add(name);
     }
-    return yield* loadCjsOrMjsDefault(
+    return yield* loadCodeDefault(
       name,
       `You appear to be using a native ECMAScript module ${type}, ` +
         "which is only supported when running Babel asynchronously.",

packages/babel-core/src/errors/config-error.ts

@@ -1,9 +1,9 @@
-import { injcectVirtualStackFrame, expectedError } from "./rewrite-stack-trace";
+import { injectVirtualStackFrame, expectedError } from "./rewrite-stack-trace";
 
 export default class ConfigError extends Error {
   constructor(message: string, filename?: string) {
     super(message);
     expectedError(this);
-    if (filename) injcectVirtualStackFrame(this, filename);
+    if (filename) injectVirtualStackFrame(this, filename);
   }
 }

packages/babel-core/src/errors/rewrite-stack-trace.ts

@@ -1,5 +1,5 @@
 /**
- * This file uses the iternal V8 Stack Trace API (https://v8.dev/docs/stack-trace-api)
+ * This file uses the internal V8 Stack Trace API (https://v8.dev/docs/stack-trace-api)
  * to provide utilities to rewrite the stack trace.
  * When this API is not present, all the functions in this file become noops.
  *
@@ -33,10 +33,10 @@
  * - If e() throws an error, then its shown call stack will be "e, f"
  *
  * Additionally, an error can inject additional "virtual" stack frames using the
- * injcectVirtualStackFrame(error, filename) function: those are injected as a
+ * injectVirtualStackFrame(error, filename) function: those are injected as a
  * replacement of the hidden frames.
- * In the example above, if we called injcectVirtualStackFrame(err, "h") and
- * injcectVirtualStackFrame(err, "i") on the expected error thrown by c(), its
+ * In the example above, if we called injectVirtualStackFrame(err, "h") and
+ * injectVirtualStackFrame(err, "i") on the expected error thrown by c(), its
  * shown call stack would have been "h, i, e, f".
  * This can be useful, for example, to report config validation errors as if they
  * were directly thrown in the config file.
@@ -46,8 +46,8 @@ const ErrorToString = Function.call.bind(Error.prototype.toString);
 
 const SUPPORTED = !!Error.captureStackTrace;
 
-const START_HIDNG = "startHiding - secret - don't use this - v1";
-const STOP_HIDNG = "stopHiding - secret - don't use this - v1";
+const START_HIDING = "startHiding - secret - don't use this - v1";
+const STOP_HIDING = "stopHiding - secret - don't use this - v1";
 
 type CallSite = Parameters<typeof Error.prepareStackTrace>[1][number];
 
@@ -70,7 +70,7 @@ function CallSite(filename: string): CallSite {
   } as CallSite);
 }
 
-export function injcectVirtualStackFrame(error: Error, filename: string) {
+export function injectVirtualStackFrame(error: Error, filename: string) {
   if (!SUPPORTED) return;
 
   let frames = virtualFrames.get(error);
@@ -97,7 +97,7 @@ export function beginHiddenCallStack<A extends unknown[], R>(
       return fn(...args);
     },
     "name",
-    { value: STOP_HIDNG },
+    { value: STOP_HIDING },
   );
 }
 
@@ -111,7 +111,7 @@ export function endHiddenCallStack<A extends unknown[], R>(
       return fn(...args);
     },
     "name",
-    { value: START_HIDNG },
+    { value: START_HIDING },
   );
 }
 
@@ -144,9 +144,9 @@ function setupPrepareStackTrace() {
       : "unknown";
     for (let i = 0; i < trace.length; i++) {
       const name = trace[i].getFunctionName();
-      if (name === START_HIDNG) {
+      if (name === START_HIDING) {
         status = "hiding";
-      } else if (name === STOP_HIDNG) {
+      } else if (name === STOP_HIDING) {
         if (status === "hiding") {
           status = "showing";
           if (virtualFrames.has(err)) {