[babel 8] Move @babel/register transform to a separate worker (#14025)

packages/babel-node/test/fixtures/cli/--config-file/in-files/index.js

@@ -1 +1,3 @@
-console.log("foo");
+// See https://github.com/babel/babel/pull/14025#issuecomment-986296424
+// for the reason behind using setImmediate.
+setImmediate(() => console.log("foo"));

packages/babel-node/test/fixtures/cli/--env-name/in-files/index.js

@@ -1 +1,3 @@
-console.log("foo");
+// See https://github.com/babel/babel/pull/14025#issuecomment-986296424
+// for the reason behind using setImmediate.
+setImmediate(() => console.log("foo"));

packages/babel-node/test/fixtures/cli/--presets/in-files/index.js

@@ -1 +1,3 @@
-console.log("foo");
+// See https://github.com/babel/babel/pull/14025#issuecomment-986296424
+// for the reason behind using setImmediate.
+setImmediate(() => console.log("foo"));

packages/babel-node/test/fixtures/cli/-b/in-files/index.js

@@ -1 +1,3 @@
-console.log("foo");
+// See https://github.com/babel/babel/pull/14025#issuecomment-986296424
+// for the reason behind using setImmediate.
+setImmediate(() => console.log("foo"));

packages/babel-register/.npmignore

@@ -1,3 +1,7 @@
 src
 test
 *.log
+
+experimental-worker.js
+lib/experimental-worker.js
+lib/is-in-register-worker.js

packages/babel-register/experimental-worker.js

@@ -0,0 +1 @@
+module.exports = require("./lib/experimental-worker");

packages/babel-register/package.json

@@ -12,9 +12,10 @@
     "directory": "packages/babel-register"
   },
   "author": "The Babel Team (https://babel.dev/team)",
+  "type": "commonjs",
   "main": "./lib/index.js",
   "browser": {
-    "./lib/nodeWrapper.js": "./lib/browser.js"
+    "./lib/index.js": "./lib/browser.js"
   },
   "dependencies": {
     "clone-deep": "^4.0.1",
@@ -28,6 +29,7 @@
   },
   "devDependencies": {
     "@babel/core": "workspace:^",
+    "@babel/plugin-transform-arrow-functions": "workspace:^",
     "@babel/plugin-transform-modules-commonjs": "workspace:^",
     "browserify": "^16.5.2"
   },

packages/babel-register/src/browser.js

@@ -0,0 +1,10 @@
+// required to safely use babel/register within a browserify codebase
+
+function register() {}
+
+module.exports = Object.assign(register, {
+  default: register,
+  register,
+  revert: function revert() {},
+  __esModule: true,
+});

packages/babel-register/src/browser.ts

@@ -1,5 +0,0 @@
-// required to safely use babel/register within a browserify codebase
-
-export default function register() {}
-
-export function revert() {}

packages/babel-register/src/cache.js

@@ -0,0 +1,3 @@
+// File moved to ./worker/cache.js
+// TODO: Remove this backward-compat "proxy file" in Babel 8
+module.exports = require("./worker/cache");

packages/babel-register/src/experimental-worker.js

@@ -0,0 +1,26 @@
+// TODO: Move this file to index.js in Babel 8
+
+"use strict";
+
+const [major, minor] = process.versions.node.split(".").map(Number);
+
+if (major < 12 || (major === 12 && minor < 3)) {
+  throw new Error(
+    "@babel/register/experimental-worker requires Node.js >= 12.3.0",
+  );
+}
+
+const hook = require("./hook");
+const { WorkerClient } = require("./worker-client");
+
+const register = hook.register.bind(null, new WorkerClient());
+
+module.exports = Object.assign(register, {
+  revert: hook.revert,
+  default: register,
+  __esModule: true,
+});
+
+if (!require("./is-in-register-worker").isInRegisterWorker) {
+  register();
+}

packages/babel-register/src/hook.js

@@ -0,0 +1,85 @@
+"use strict";
+
+const { addHook } = require("pirates");
+const sourceMapSupport = require("source-map-support");
+
+let piratesRevert;
+const maps = Object.create(null);
+
+function installSourceMapSupport() {
+  installSourceMapSupport = () => {}; // eslint-disable-line no-func-assign
+
+  sourceMapSupport.install({
+    handleUncaughtExceptions: false,
+    environment: "node",
+    retrieveSourceMap(filename) {
+      const map = maps?.[filename];
+      if (map) {
+        return { url: null, map: map };
+      } else {
+        return null;
+      }
+    },
+  });
+}
+
+if (!process.env.BABEL_8_BREAKING) {
+  // Babel 7 compiles files in the same thread where it hooks `require()`,
+  // so we must prevent mixing Babel plugin dependencies with the files
+  // to be compiled.
+  // All the `!process.env.BABEL_8_BREAKING` code in this file is for
+  // this purpose.
+
+  const Module = require("module");
+
+  let compiling = false;
+  const internalModuleCache = Module._cache;
+
+  // eslint-disable-next-line no-var
+  var compileBabel7 = function compileBabel7(client, code, filename) {
+    if (!client.isLocalClient) return compile(client, code, filename);
+
+    if (compiling) return code;
+
+    const globalModuleCache = Module._cache;
+    try {
+      compiling = true;
+      Module._cache = internalModuleCache;
+      return compile(client, code, filename);
+    } finally {
+      compiling = false;
+      Module._cache = globalModuleCache;
+    }
+  };
+}
+
+function compile(client, inputCode, filename) {
+  const result = client.transform(inputCode, filename);
+
+  if (result === null) return inputCode;
+
+  const { code, map } = result;
+  if (map) {
+    maps[filename] = map;
+    installSourceMapSupport();
+  }
+  return code;
+}
+
+exports.register = function register(client, opts = {}) {
+  if (piratesRevert) piratesRevert();
+
+  piratesRevert = addHook(
+    (process.env.BABEL_8_BREAKING ? compile : compileBabel7).bind(null, client),
+    {
+      exts: opts.extensions ?? client.getDefaultExtensions(),
+      ignoreNodeModules: false,
+    },
+  );
+
+  client.setOptions(opts);
+};
+
+exports.revert = function revert() {
+  if (piratesRevert) piratesRevert();
+};

packages/babel-register/src/index.js

@@ -4,12 +4,16 @@
  * from a compiled Babel import.
  */
 
-exports = module.exports = function (...args) {
+if (process.env.BABEL_8_BREAKING) {
+  module.exports = require("./experimental-worker");
+} else {
+  exports = module.exports = function (...args) {
     return register(...args);
-};
-exports.__esModule = true;
+  };
+  exports.__esModule = true;
 
-const node = require("./nodeWrapper");
-const register = node.default;
+  const node = require("./nodeWrapper");
+  const register = node.default;
 
-Object.assign(exports, node);
+  Object.assign(exports, node);
+}

packages/babel-register/src/is-in-register-worker.js

@@ -0,0 +1,20 @@
+"use strict";
+
+/**
+ * Since workers inherit the exec options from the parent thread, we
+ * must be careful to avoid infite "@babel/register" setup loops.
+ *
+ * If @babel/register is imported using the -r/--require flag, the worker
+ * will have the same flag and we must avoid registering the @babel/register
+ * hook again.
+ *
+ * - markInRegisterWorker() can be used to mark a set of env vars (that will
+ *   be forwarded to a worker) as being in the @babel/register worker.
+ * - isInRegisterWorker will be true in @babel/register workers.
+ */
+
+const envVarName = "___INTERNAL___IS_INSIDE_BABEL_REGISTER_WORKER___";
+const envVarValue = "yes_I_am";
+
+exports.markInRegisterWorker = env => ({ ...env, [envVarName]: envVarValue });
+exports.isInRegisterWorker = process.env[envVarName] === envVarValue;

packages/babel-register/src/node.js

@@ -0,0 +1,13 @@
+// TODO: Remove this file in Babel 8
+
+"use strict";
+
+const hook = require("./hook");
+const { LocalClient } = require("./worker-client");
+
+const register = hook.register.bind(null, new LocalClient());
+
+module.exports = Object.assign(register, {
+  revert: hook.revert,
+  default: register,
+});

packages/babel-register/src/node.ts

@@ -1,176 +0,0 @@
-import cloneDeep from "clone-deep";
-import sourceMapSupport from "source-map-support";
-import * as registerCache from "./cache";
-import * as babel from "@babel/core";
-import { OptionManager, DEFAULT_EXTENSIONS } from "@babel/core";
-import { addHook } from "pirates";
-import fs from "fs";
-import path from "path";
-import Module from "module";
-
-const maps = {};
-let transformOpts: any = {};
-let piratesRevert = null;
-
-function installSourceMapSupport() {
-  sourceMapSupport.install({
-    handleUncaughtExceptions: false,
-    environment: "node",
-    retrieveSourceMap(source) {
-      const map = maps && maps[source];
-      if (map) {
-        return {
-          url: null,
-          map: map,
-        };
-      } else {
-        return null;
-      }
-    },
-  });
-}
-
-let cache;
-
-function mtime(filename) {
-  return +fs.statSync(filename).mtime;
-}
-
-function compile(code, filename) {
-  // merge in base options and resolve all the plugins and presets relative to this file
-  const opts = new OptionManager().init(
-    // sourceRoot can be overwritten
-    {
-      sourceRoot: path.dirname(filename) + path.sep,
-      ...cloneDeep(transformOpts),
-      filename,
-    },
-  );
-
-  // Bail out ASAP if the file has been ignored.
-  if (opts === null) return code;
-
-  let cacheKey = `${JSON.stringify(opts)}:${babel.version}`;
-
-  const env = babel.getEnv("");
-
-  if (env) cacheKey += `:${env}`;
-
-  let cached, fileMtime;
-  if (cache) {
-    cached = cache[cacheKey];
-    fileMtime = mtime(filename);
-  }
-
-  if (!cached || cached.mtime !== fileMtime) {
-    cached = babel.transform(code, {
-      ...opts,
-      sourceMaps: opts.sourceMaps === undefined ? "both" : opts.sourceMaps,
-      ast: false,
-    });
-
-    if (cache) {
-      cache[cacheKey] = cached;
-      cached.mtime = fileMtime;
-      registerCache.setDirty();
-    }
-  }
-
-  if (cached.map) {
-    if (Object.keys(maps).length === 0) {
-      installSourceMapSupport();
-    }
-    maps[filename] = cached.map;
-  }
-
-  return cached.code;
-}
-
-let compiling = false;
-// @ts-expect-error field is missing in type definitions
-const internalModuleCache = Module._cache;
-
-function compileHook(code, filename) {
-  if (compiling) return code;
-
-  // @ts-expect-error field is missing in type definitions
-  const globalModuleCache = Module._cache;
-  try {
-    compiling = true;
-    // @ts-expect-error field is missing in type definitions
-    Module._cache = internalModuleCache;
-    return compile(code, filename);
-  } finally {
-    compiling = false;
-    // @ts-expect-error field is missing in type definitions
-    Module._cache = globalModuleCache;
-  }
-}
-
-function hookExtensions(exts) {
-  if (piratesRevert) piratesRevert();
-  piratesRevert = addHook(compileHook, { exts, ignoreNodeModules: false });
-}
-
-export function revert() {
-  if (piratesRevert) piratesRevert();
-}
-
-function escapeRegExp(string) {
-  return string.replace(/[|\\{}()[\]^$+*?.]/g, "\\$&");
-}
-
-export default function register(opts: any = {}) {
-  // Clone to avoid mutating the arguments object with the 'delete's below.
-  opts = {
-    ...opts,
-  };
-  hookExtensions(opts.extensions || DEFAULT_EXTENSIONS);
-
-  if (opts.cache === false && cache) {
-    registerCache.clear();
-    cache = null;
-  } else if (opts.cache !== false && !cache) {
-    registerCache.load();
-    cache = registerCache.get();
-  }
-
-  delete opts.extensions;
-  delete opts.cache;
-
-  transformOpts = {
-    ...opts,
-    caller: {
-      name: "@babel/register",
-      ...(opts.caller || {}),
-    },
-  };
-
-  let { cwd = "." } = transformOpts;
-
-  // Ensure that the working directory is resolved up front so that
-  // things don't break if it changes later.
-  cwd = transformOpts.cwd = path.resolve(cwd);
-
-  if (transformOpts.ignore === undefined && transformOpts.only === undefined) {
-    transformOpts.only = [
-      // Only compile things inside the current working directory.
-      // $FlowIgnore
-      new RegExp("^" + escapeRegExp(cwd), "i"),
-    ];
-    transformOpts.ignore = [
-      // Ignore any node_modules inside the current working directory.
-      new RegExp(
-        "^" +
-          // $FlowIgnore
-          escapeRegExp(cwd) +
-          "(?:" +
-          path.sep +
-          ".*)?" +
-          // $FlowIgnore
-          escapeRegExp(path.sep + "node_modules" + path.sep),
-        "i",
-      ),
-    ];
-  }
-}

packages/babel-register/src/nodeWrapper.js

@@ -5,7 +5,8 @@
  * and allows register to transform these modules if they are loaded externally.
  */
 
-// @ts-ignore todo(flow->ts) convert to esm
+// TODO: Remove this file in Babel 8
+
 const Module = require("module");
 
 const globalModuleCache = Module._cache;

packages/babel-register/src/worker-client.js

@@ -0,0 +1,110 @@
+const path = require("path");
+
+const ACTIONS = {
+  GET_DEFAULT_EXTENSIONS: "GET_DEFAULT_EXTENSIONS",
+  SET_OPTIONS: "SET_OPTIONS",
+  TRANSFORM: "TRANSFORM",
+  TRANSFORM_SYNC: "TRANSFORM_SYNC",
+};
+
+class Client {
+  #send;
+
+  constructor(send) {
+    this.#send = send;
+  }
+
+  #eCache;
+  /** @return {string[]} */
+  getDefaultExtensions() {
+    return (this.#eCache ??= this.#send(
+      ACTIONS.GET_DEFAULT_EXTENSIONS,
+      undefined,
+    ));
+  }
+
+  /**
+   * @param {object} options
+   * @return {void}
+   */
+  setOptions(options) {
+    return this.#send(ACTIONS.SET_OPTIONS, options);
+  }
+
+  /**
+   * @param {string} code
+   * @param {string} filename
+   * @return {{ code: string, map: object } | null}
+   */
+  transform(code, filename) {
+    return this.#send(ACTIONS.TRANSFORM, { code, filename });
+  }
+}
+
+// We need to run Babel in a worker because require hooks must
+// run synchronously, but many steps of Babel's config loading
+// (which is done for each file) can be asynchronous
+exports.WorkerClient = class WorkerClient extends Client {
+  // These two require() calls are in deferred so that they are not imported in
+  // older Node.js versions (which don't support workers).
+  // TODO: Hoist them in Babel 8.
+
+  /** @type {typeof import("worker_threads")} */
+  static get #worker_threads() {
+    return require("worker_threads");
+  }
+
+  static get #markInRegisterWorker() {
+    return require("./is-in-register-worker").markInRegisterWorker;
+  }
+
+  #worker = new WorkerClient.#worker_threads.Worker(
+    path.resolve(__dirname, "./worker/index.js"),
+    { env: WorkerClient.#markInRegisterWorker(process.env) },
+  );
+
+  #signal = new Int32Array(new SharedArrayBuffer(4));
+
+  constructor() {
+    super((action, payload) => {
+      this.#signal[0] = 0;
+      const subChannel = new WorkerClient.#worker_threads.MessageChannel();
+
+      this.#worker.postMessage(
+        { signal: this.#signal, port: subChannel.port1, action, payload },
+        [subChannel.port1],
+      );
+
+      Atomics.wait(this.#signal, 0, 0);
+      const { message } = WorkerClient.#worker_threads.receiveMessageOnPort(
+        subChannel.port2,
+      );
+
+      if (message.error) throw Object.assign(message.error, message.errorData);
+      else return message.result;
+    });
+
+    // The worker will never exit by itself. Prevent it from keeping
+    // the main process alive.
+    this.#worker.unref();
+  }
+};
+
+if (!process.env.BABEL_8_BREAKING) {
+  exports.LocalClient = class LocalClient extends Client {
+    isLocalClient = true;
+
+    static #handleMessage;
+
+    constructor() {
+      LocalClient.#handleMessage ??= require("./worker/handle-message");
+
+      super((action, payload) => {
+        return LocalClient.#handleMessage(
+          action === ACTIONS.TRANSFORM ? ACTIONS.TRANSFORM_SYNC : action,
+          payload,
+        );
+      });
+    }
+  };
+}

packages/babel-register/src/worker/babel-core.js

@@ -0,0 +1,20 @@
+function initialize(babel) {
+  exports.init = null;
+  exports.version = babel.version;
+  exports.DEFAULT_EXTENSIONS = babel.DEFAULT_EXTENSIONS;
+  exports.loadOptionsAsync = babel.loadOptionsAsync;
+  exports.transformAsync = babel.transformAsync;
+  exports.getEnv = babel.getEnv;
+
+  if (!process.env.BABEL_8_BREAKING) {
+    exports.loadOptionsSync = babel.loadOptionsSync;
+    exports.transformSync = babel.transformSync;
+  }
+}
+
+if (process.env.BABEL_8_BREAKING) {
+  // @ts-expect-error CJS-ESM interop.
+  exports.init = import("@babel/core").then(ns => initialize(ns.default));
+} else {
+  initialize(require("@babel/core"));
+}

packages/babel-register/src/worker/cache.js

@@ -1,8 +1,10 @@
-import path from "path";
-import fs from "fs";
-import os from "os";
-import * as babel from "@babel/core";
-import findCacheDir from "find-cache-dir";
+"use strict";
+
+const path = require("path");
+const fs = require("fs");
+const os = require("os");
+const babel = require("@babel/core");
+const findCacheDir = require("find-cache-dir");
 
 const DEFAULT_CACHE_DIR =
   findCacheDir({ name: "@babel/register" }) || os.homedir() || os.tmpdir();
@@ -10,8 +12,9 @@ const DEFAULT_FILENAME = path.join(
   DEFAULT_CACHE_DIR,
   `.babel.${babel.version}.${babel.getEnv()}.json`,
 );
-const FILENAME: string = process.env.BABEL_CACHE_PATH || DEFAULT_FILENAME;
-let data: any = {};
+
+const FILENAME = process.env.BABEL_CACHE_PATH || DEFAULT_FILENAME;
+let data = {};
 
 let cacheDirty = false;
 
@@ -20,15 +23,16 @@ let cacheDisabled = false;
 function isCacheDisabled() {
   return process.env.BABEL_DISABLE_CACHE ?? cacheDisabled;
 }
+
+exports.save = save;
 /**
  * Write stringified cache to disk.
  */
-
-export function save() {
+function save() {
   if (isCacheDisabled() || !cacheDirty) return;
   cacheDirty = false;
 
-  let serialised: string = "{}";
+  let serialised = "{}";
 
   try {
     serialised = JSON.stringify(data, null, "  ");
@@ -74,7 +78,7 @@ because it resides in a readonly filesystem. Cache is disabled.`,
  * Load cache from disk and parse.
  */
 
-export function load() {
+exports.load = function load() {
   if (isCacheDisabled()) {
     data = {};
     return;
@@ -106,27 +110,25 @@ due to a permission issue. Cache is disabled.`,
   try {
     data = JSON.parse(cacheContent);
   } catch {}
-}
+};
 
 /**
  * Retrieve data from cache.
  */
-
-export function get(): any {
+exports.get = function get() {
   return data;
-}
+};
 
 /**
  * Set the cache dirty bit.
  */
-export function setDirty() {
+exports.setDirty = function setDirty() {
   cacheDirty = true;
-}
+};
 
 /**
  * Clear the cache object.
  */
-
-export function clear() {
+exports.clear = function clear() {
   data = {};
-}
+};

packages/babel-register/src/worker/handle-message.js

@@ -0,0 +1,20 @@
+const babel = require("./babel-core");
+const { setOptions, transform, transformSync } = require("./transform");
+
+module.exports = function handleMessage(action, payload) {
+  switch (action) {
+    case "GET_DEFAULT_EXTENSIONS":
+      return babel.DEFAULT_EXTENSIONS;
+    case "SET_OPTIONS":
+      setOptions(payload);
+      return;
+    case "TRANSFORM":
+      return transform(payload.code, payload.filename);
+    case "TRANSFORM_SYNC":
+      if (!process.env.BABEL_8_BREAKING) {
+        return transformSync(payload.code, payload.filename);
+      }
+  }
+
+  throw new Error(`Unknown internal parser worker action: ${action}`);
+};

packages/babel-register/src/worker/index.js

@@ -0,0 +1,28 @@
+const babel = require("./babel-core");
+const handleMessage = require("./handle-message");
+
+const { parentPort } = require("worker_threads");
+
+parentPort.addListener("message", async ({ signal, port, action, payload }) => {
+  let response;
+
+  try {
+    if (babel.init) await babel.init;
+
+    response = { result: await handleMessage(action, payload) };
+  } catch (error) {
+    response = { error, errorData: { ...error } };
+  }
+
+  try {
+    port.postMessage(response);
+  } catch {
+    port.postMessage({
+      error: new Error("Cannot serialize worker response"),
+    });
+  } finally {
+    port.close();
+    Atomics.store(signal, 0, 1);
+    Atomics.notify(signal, 0);
+  }
+});

packages/babel-register/src/worker/transform.js

@@ -0,0 +1,128 @@
+"use strict";
+
+const cloneDeep = require("clone-deep");
+const path = require("path");
+const fs = require("fs");
+
+const babel = require("./babel-core");
+const registerCache = require("../cache");
+
+const nmRE = escapeRegExp(path.sep + "node_modules" + path.sep);
+
+function escapeRegExp(string) {
+  return string.replace(/[|\\{}()[\]^$+*?.]/g, "\\$&");
+}
+
+let cache;
+let transformOpts;
+exports.setOptions = function (opts) {
+  if (opts.cache === false && cache) {
+    registerCache.clear();
+    cache = null;
+  } else if (opts.cache !== false && !cache) {
+    registerCache.load();
+    cache = registerCache.get();
+  }
+
+  delete opts.cache;
+  delete opts.extensions;
+
+  transformOpts = {
+    ...opts,
+    caller: {
+      name: "@babel/register",
+      ...(opts.caller || {}),
+    },
+  };
+
+  let { cwd = "." } = transformOpts;
+
+  // Ensure that the working directory is resolved up front so that
+  // things don't break if it changes later.
+  cwd = transformOpts.cwd = path.resolve(cwd);
+
+  if (transformOpts.ignore === undefined && transformOpts.only === undefined) {
+    const cwdRE = escapeRegExp(cwd);
+
+    // Only compile things inside the current working directory.
+    transformOpts.only = [new RegExp("^" + cwdRE, "i")];
+    // Ignore any node_modules inside the current working directory.
+    transformOpts.ignore = [
+      new RegExp(`^${cwdRE}(?:${path.sep}.*)?${nmRE}`, "i"),
+    ];
+  }
+};
+
+exports.transform = async function (input, filename) {
+  const opts = await babel.loadOptionsAsync({
+    // sourceRoot can be overwritten
+    sourceRoot: path.dirname(filename) + path.sep,
+    ...cloneDeep(transformOpts),
+    filename,
+  });
+
+  // Bail out ASAP if the file has been ignored.
+  if (opts === null) return null;
+
+  const { cached, store } = cacheLookup(opts, filename);
+  if (cached) return cached;
+
+  const { code, map } = await babel.transformAsync(input, {
+    ...opts,
+    sourceMaps: opts.sourceMaps === undefined ? "both" : opts.sourceMaps,
+    ast: false,
+  });
+
+  return store({ code, map });
+};
+
+if (!process.env.BABEL_8_BREAKING) {
+  exports.transformSync = function (input, filename) {
+    const opts = babel.loadOptionsSync({
+      // sourceRoot can be overwritten
+      sourceRoot: path.dirname(filename) + path.sep,
+      ...cloneDeep(transformOpts),
+      filename,
+    });
+
+    // Bail out ASAP if the file has been ignored.
+    if (opts === null) return null;
+
+    const { cached, store } = cacheLookup(opts, filename);
+    if (cached) return cached;
+
+    const { code, map } = babel.transformSync(input, {
+      ...opts,
+      sourceMaps: opts.sourceMaps === undefined ? "both" : opts.sourceMaps,
+      ast: false,
+    });
+
+    return store({ code, map });
+  };
+}
+
+const id = value => value;
+
+function cacheLookup(opts, filename) {
+  if (!cache) return { cached: null, store: id };
+
+  let cacheKey = `${JSON.stringify(opts)}:${babel.version}`;
+
+  const env = babel.getEnv();
+  if (env) cacheKey += `:${env}`;
+
+  const cached = cache[cacheKey];
+  const fileMtime = +fs.statSync(filename).mtime;
+
+  if (cached && cached.mtime === fileMtime) {
+    return { cached: cached.value, store: id };
+  }
+
+  return {
+    cached: null,
+    store(value) {
+      cache[cacheKey] = { value, mtime: fileMtime };
+      return value;
+    },
+  };
+}

packages/babel-register/test/fixtures/babelrc/.babelrc

@@ -1,5 +1,6 @@
 {
     "plugins": [
-        "@babel/transform-modules-commonjs"
+        "@babel/transform-modules-commonjs",
+        "@babel/plugin-transform-arrow-functions"
     ]
 }

packages/babel-register/test/fixtures/babelrc/log.js

@@ -0,0 +1 @@
+console.log("It worked!", (() => {}).toString());

packages/babel-register/test/fixtures/internal-modules/index.js

@@ -1,23 +1,10 @@
 const register = require('../../..');
 
-// Plugin to add '/* transformed */' comment to start of function bodies
-const plugin = () => ( {
-  visitor: {
-    Function(path) {
-      const bodyNode = path.node.body;
-      (bodyNode.leadingComments || (bodyNode.leadingComments = [])).push( {
-        type: 'CommentBlock',
-        value: ' transformed '
-      } );
-    },
-  },
-} );
-
 register( {
   ignore: [],
   babelrc: false,
   configFile: false,
-  plugins: [plugin]
+  plugins: [require.resolve("./plugin")]
 } );
 
 console.log(

packages/babel-register/test/fixtures/internal-modules/plugin.js

@@ -0,0 +1,13 @@
+// Plugin to add '/* transformed */' comment to start of function bodies
+
+module.exports = () => ( {
+  visitor: {
+    Function(path) {
+      const bodyNode = path.node.body;
+      (bodyNode.leadingComments || (bodyNode.leadingComments = [])).push( {
+        type: 'CommentBlock',
+        value: ' transformed '
+      } );
+    },
+  },
+} );

packages/babel-register/test/fixtures/mjs-babelrc/.babelrc.mjs

@@ -0,0 +1,3 @@
+export default {
+  plugins: ["@babel/transform-modules-commonjs"],
+};

packages/babel-register/test/fixtures/mjs-babelrc/es2015.js

@@ -0,0 +1 @@
+import "assert";

packages/babel-register/test/index.js

@@ -7,10 +7,12 @@ import { fileURLToPath } from "url";
 const dirname = path.dirname(fileURLToPath(import.meta.url));
 const require = createRequire(import.meta.url);
 
-const registerFile = require.resolve("../lib/index");
 const testCacheFilename = path.join(dirname, ".index.babel");
 const testFile = require.resolve("./fixtures/babelrc/es2015");
-const testFileContent = fs.readFileSync(testFile);
+const testFileLog = require.resolve("./fixtures/babelrc/log");
+const testFileMjs = require.resolve("./fixtures/mjs-babelrc/es2015");
+const testFileContent = fs.readFileSync(testFile, "utf-8");
+const testFileMjsContent = fs.readFileSync(testFileMjs, "utf-8");
 
 const piratesPath = require.resolve("pirates");
 const smsPath = require.resolve("source-map-support");
@@ -57,48 +59,13 @@ describe("@babel/register", function () {
     },
   };
 
-  let babelRegister;
-
-  function setupRegister(config = { babelrc: false }) {
-    process.env.BABEL_CACHE_PATH = testCacheFilename;
-    config = {
-      cwd: path.dirname(testFile),
-      ...config,
-    };
-
-    babelRegister = require(registerFile);
-    babelRegister.default(config);
-  }
-
-  function revertRegister() {
-    if (babelRegister) {
-      babelRegister.revert();
-      delete require.cache[registerFile];
-      babelRegister = null;
-    }
-    cleanCache();
-  }
-
   beforeEach(() => {
     currentHook = null;
     currentOptions = null;
     sourceMapSupport = false;
   });
 
-  afterEach(async () => {
-    // @babel/register saves the cache on process.nextTick.
-    // We need to wait for at least one tick so that when jest
-    // tears down the testing environment @babel/register has
-    // already finished.
-    await new Promise(setImmediate);
-
-    revertRegister();
-  });
-
-  afterAll(() => {
-    resetCache();
-  });
-
+  let originalRequireCacheDescriptor;
   if (OLD_JEST_MOCKS) {
     jest.doMock("pirates", () => mocks["pirates"]);
     jest.doMock("source-map-support", () => mocks["source-map-support"]);
@@ -107,7 +74,6 @@ describe("@babel/register", function () {
       jest.resetModules();
     });
   } else {
-    let originalRequireCacheDescriptor;
     beforeAll(() => {
       originalRequireCacheDescriptor = Object.getOwnPropertyDescriptor(
         Module,
@@ -115,9 +81,18 @@ describe("@babel/register", function () {
       );
     });
 
+    afterAll(() => {
+      Object.defineProperty(Module, "_cache", originalRequireCacheDescriptor);
+    });
+  }
+
+  if (!process.env.BABEL_8_BREAKING) {
+    describe("babel 7", () => {
+      if (!OLD_JEST_MOCKS) {
         beforeEach(() => {
           const isEmptyObj = obj =>
-        Object.getPrototypeOf(obj) === null && Object.keys(obj).length === 0;
+            Object.getPrototypeOf(obj) === null &&
+            Object.keys(obj).length === 0;
 
           // This setter intercepts the Module._cache assignment in
           // packages/babel-register/src/nodeWrapper.js to install in the
@@ -141,12 +116,85 @@ describe("@babel/register", function () {
             configurable: originalRequireCacheDescriptor.configurable,
           });
         });
+      }
 
-    afterAll(() => {
-      Object.defineProperty(Module, "_cache", originalRequireCacheDescriptor);
+      buildTests(require.resolve(".."));
     });
   }
 
+  const nodeGte12 = (fn, ...args) => {
+    // "minNodeVersion": "8.0.0" <-- For Ctrl+F when dropping node 6-8-10
+    const testFn = /v(?:6|8|10)\./.test(process.version) ? fn.skip : fn;
+    testFn(...args);
+  };
+
+  nodeGte12(describe, "worker", () => {
+    if (!OLD_JEST_MOCKS) {
+      beforeEach(() => {
+        Object.defineProperty(Module, "_cache", {
+          ...originalRequireCacheDescriptor,
+          value: {
+            [piratesPath]: { exports: mocks["pirates"] },
+            [smsPath]: { exports: mocks["source-map-support"] },
+          },
+        });
+      });
+    }
+
+    const { setupRegister } = buildTests(
+      require.resolve("../experimental-worker"),
+    );
+
+    it("works with mjs config files", () => {
+      setupRegister({
+        babelrc: true,
+        sourceMaps: false,
+        cwd: path.dirname(testFileMjs),
+      });
+
+      const result = currentHook(testFileMjsContent, testFileMjs);
+
+      expect(result).toBe('"use strict";\n\nrequire("assert");');
+    });
+  });
+
+  function buildTests(registerFile) {
+    let babelRegister;
+
+    function setupRegister(config = { babelrc: false }) {
+      process.env.BABEL_CACHE_PATH = testCacheFilename;
+      config = {
+        cwd: path.dirname(testFile),
+        ...config,
+      };
+
+      babelRegister = require(registerFile);
+      babelRegister.default(config);
+    }
+
+    function revertRegister() {
+      if (babelRegister) {
+        babelRegister.revert();
+        delete require.cache[registerFile];
+        babelRegister = null;
+      }
+      cleanCache();
+    }
+
+    afterEach(async () => {
+      // @babel/register saves the cache on process.nextTick.
+      // We need to wait for at least one tick so that when jest
+      // tears down the testing environment @babel/register has
+      // already finished.
+      await new Promise(setImmediate);
+
+      revertRegister();
+    });
+
+    afterAll(() => {
+      resetCache();
+    });
+
     test("registers hook correctly", () => {
       setupRegister();
 
@@ -192,6 +240,54 @@ describe("@babel/register", function () {
       expect(sourceMapSupport).toBe(false);
     });
 
+    describe("node auto-require", () => {
+      it("works with the -r flag", async () => {
+        const output = await spawnNodeAsync(
+          ["-r", registerFile, testFileLog],
+          path.dirname(testFileLog),
+        );
+
+        expect(output.trim()).toMatchInlineSnapshot(
+          `"It worked! function () {}"`,
+        );
+      });
+
+      it("works with the --require flag", async () => {
+        const output = await spawnNodeAsync(
+          ["--require", registerFile, testFileLog],
+          path.dirname(testFileLog),
+        );
+
+        expect(output.trim()).toMatchInlineSnapshot(
+          `"It worked! function () {}"`,
+        );
+      });
+
+      it("works with the -r flag in NODE_OPTIONS", async () => {
+        const output = await spawnNodeAsync(
+          [testFileLog],
+          path.dirname(testFileLog),
+          { NODE_OPTIONS: `-r ${registerFile}` },
+        );
+
+        expect(output.trim()).toMatchInlineSnapshot(
+          `"It worked! function () {}"`,
+        );
+      });
+
+      it("works with the --require flag in NODE_OPTIONS", async () => {
+        const output = await spawnNodeAsync(
+          [testFileLog],
+          path.dirname(testFileLog),
+          { NODE_OPTIONS: `--require ${registerFile}` },
+        );
+
+        expect(output.trim()).toMatchInlineSnapshot(
+          `"It worked! function () {}"`,
+        );
+      });
+    });
+
     it("returns concatenatable sourceRoot and sources", async () => {
       // The Source Maps R3 standard https://sourcemaps.info/spec.html states
       // that `sourceRoot` is “prepended to the individual entries in the
@@ -247,10 +343,13 @@ describe("@babel/register", function () {
       const { convertSourceMap } = JSON.parse(output);
       expect(convertSourceMap).toMatch("/* transformed */");
     });
+
+    return { setupRegister, revertRegister };
+  }
 });
 
-function spawnNodeAsync(args) {
-  const spawn = child.spawn(process.execPath, args, { cwd: dirname });
+function spawnNodeAsync(args, cwd = dirname, env) {
+  const spawn = child.spawn(process.execPath, args, { cwd, env });
 
   let output = "";
   let callback;

yarn.lock

@@ -3494,6 +3494,7 @@ __metadata:
   resolution: "@babel/register@workspace:packages/babel-register"
   dependencies:
     "@babel/core": "workspace:^"
+    "@babel/plugin-transform-arrow-functions": "workspace:^"
     "@babel/plugin-transform-modules-commonjs": "workspace:^"
     browserify: ^16.5.2
     clone-deep: ^4.0.1