Improve CJS compat with ESM-based @babel/core

babel.config.js

@@ -231,6 +231,11 @@ module.exports = function (api) {
             { name: "USE_ESM", value: outputType === "module" },
             "flag-USE_ESM",
           ],
+          [
+            pluginToggleBooleanFlag,
+            { name: "IS_STANDALONE", value: env === "standalone" },
+            "flag-IS_STANDALONE",
+          ],
 
           process.env.STRIP_BABEL_8_FLAG && [
             pluginToggleBooleanFlag,

packages/babel-core/cjs-proxy.cjs

@@ -1,6 +1,12 @@
 "use strict";
 
 const babelP = import("./lib/index.js");
+let babel = null;
+Object.defineProperty(exports, "__ initialize @babel/core cjs proxy __", {
+  set(val) {
+    babel = val;
+  },
+});
 
 const functionNames = [
   "createConfigItem",
@@ -11,13 +17,9 @@ const functionNames = [
   "transformFromAst",
   "parse",
 ];
+const propertyNames = ["types", "tokTypes", "traverse", "template", "version"];
 
 for (const name of functionNames) {
-  exports[`${name}Sync`] = function () {
-    throw new Error(
-      `"${name}Sync" is not supported when loading @babel/core using require()`
-    );
-  };
   exports[name] = function (...args) {
     babelP.then(babel => {
       babel[name](...args);
@@ -26,4 +28,24 @@ for (const name of functionNames) {
   exports[`${name}Async`] = function (...args) {
     return babelP.then(babel => babel[`${name}Async`](...args));
   };
+  exports[`${name}Sync`] = function (...args) {
+    if (!babel) throw notLoadedError(`${name}Sync`, "callable");
+    return babel[`${name}Sync`](...args);
+  };
+}
+
+for (const name of propertyNames) {
+  Object.defineProperty(exports, name, {
+    get() {
+      if (!babel) throw notLoadedError(name, "accessible");
+      return babel[name];
+    },
+  });
+}
+
+function notLoadedError(name, keyword) {
+  return new Error(
+    `The \`${name}\` export of @babel/core is only ${keyword}` +
+      ` from the CommonJS version after that the ESM version is loaded.`
+  );
 }

packages/babel-core/src/index.ts

@@ -1,4 +1,6 @@
 declare const PACKAGE_JSON: { name: string; version: string };
+declare const USE_ESM: boolean, IS_STANDALONE: boolean;
+
 export const version = PACKAGE_JSON.version;
 
 export { default as File } from "./transformation/file/file";
@@ -72,15 +74,27 @@ export const DEFAULT_EXTENSIONS = Object.freeze([
 ] as const);
 
 // For easier backward-compatibility, provide an API like the one we exposed in Babel 6.
+// TODO(Babel 8): Remove this.
 import { loadOptionsSync } from "./config";
 export class OptionManager {
   init(opts: {}) {
     return loadOptionsSync(opts);
   }
 }
 
+// TODO(Babel 8): Remove this.
 export function Plugin(alias: string) {
   throw new Error(
     `The (${alias}) Babel 5 plugin is being run with an unsupported Babel version.`,
   );
 }
+
+import Module from "module";
+import * as thisFile from "./index";
+if (USE_ESM) {
+  if (!IS_STANDALONE) {
+    // Pass this module to the CJS proxy, so that it can be synchronously accessed.
+    const cjsProxy = Module.createRequire(import.meta.url)("../cjs-proxy.cjs");
+    cjsProxy["__ initialize @babel/core cjs proxy __"] = thisFile;
+  }
+}

packages/babel-core/test/esm-cjs-integration.js

@@ -0,0 +1,109 @@
+import { execFile } from "child_process";
+import { createRequire } from "module";
+import { outputType } from "./helpers/esm.js";
+
+const require = createRequire(import.meta.url);
+
+async function run(name) {
+  return new Promise((res, rej) => {
+    execFile(
+      process.execPath,
+      [require.resolve(`./fixtures/esm-cjs-integration/${name}`)],
+      { env: process.env },
+      (error, stdout, stderr) => {
+        if (error) rej(error);
+        res({ stdout: stdout.toString(), stderr: stderr.toString() });
+      },
+    );
+  });
+}
+
+(outputType === "module" ? describe : describe.skip)("usage from cjs", () => {
+  it("lazy plugin required", async () => {
+    expect(await run("lazy-plugin-required.cjs")).toMatchInlineSnapshot(`
+      Object {
+        "stderr": "",
+        "stdout": "\\"Replaced!\\";
+      ",
+      }
+    `);
+  });
+
+  it("lazy plugin as config string", async () => {
+    expect(await run("lazy-plugin-as-string.cjs")).toMatchInlineSnapshot(`
+      Object {
+        "stderr": "",
+        "stdout": "\\"Replaced!\\";
+      ",
+      }
+    `);
+  });
+
+  it("eager plugin required", async () => {
+    await expect(run("eager-plugin-required.cjs")).rejects.toThrow(
+      "The `types` export of @babel/core is only accessible from" +
+        " the CommonJS version after that the ESM version is loaded.",
+    );
+  });
+
+  it("eager plugin required after dynamic esm import", async () => {
+    expect(await run("eager-plugin-required-after-dynamic-esm-import.cjs"))
+      .toMatchInlineSnapshot(`
+        Object {
+          "stderr": "",
+          "stdout": "\\"Replaced!\\";
+        ",
+        }
+      `);
+  });
+
+  it("eager plugin required after static esm import", async () => {
+    expect(await run("eager-plugin-required-after-static-esm-import.mjs"))
+      .toMatchInlineSnapshot(`
+        Object {
+          "stderr": "",
+          "stdout": "\\"Replaced!\\";
+        ",
+        }
+      `);
+  });
+
+  it("eager plugin as config string", async () => {
+    expect(await run("eager-plugin-as-string.cjs")).toMatchInlineSnapshot(`
+      Object {
+        "stderr": "",
+        "stdout": "\\"Replaced!\\";
+      ",
+      }
+    `);
+  });
+
+  it("transformSync", async () => {
+    await expect(run("transform-sync.cjs")).rejects.toThrow(
+      "The `transformSync` export of @babel/core is only callable from" +
+        " the CommonJS version after that the ESM version is loaded.",
+    );
+  });
+
+  it("transformSync after dynamic esm import", async () => {
+    expect(await run("transform-sync-after-dynamic-esm-import.cjs"))
+      .toMatchInlineSnapshot(`
+        Object {
+          "stderr": "",
+          "stdout": "REPLACE_ME;
+        ",
+        }
+      `);
+  });
+
+  it("transformSync after static esm import", async () => {
+    expect(await run("transform-sync-after-static-esm-import.mjs"))
+      .toMatchInlineSnapshot(`
+        Object {
+          "stderr": "",
+          "stdout": "REPLACE_ME;
+        ",
+        }
+      `);
+  });
+});

packages/babel-core/test/fixtures/esm-cjs-integration/eager-plugin-as-string.cjs

@@ -0,0 +1,8 @@
+const babel = require("../../../cjs-proxy.cjs");
+
+babel
+  .transformAsync("REPLACE_ME;", {
+    configFile: false,
+    plugins: [__dirname + "/plugins/eager.cjs"],
+  })
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/eager-plugin-required-after-dynamic-esm-import.cjs

@@ -0,0 +1,8 @@
+import("../../../lib/index.js")
+  .then(babel =>
+    babel.transformAsync("REPLACE_ME;", {
+      configFile: false,
+      plugins: [require("./plugins/eager.cjs")],
+    }),
+  )
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/eager-plugin-required-after-static-esm-import.mjs

@@ -0,0 +1,8 @@
+import * as babel from "../../../lib/index.js";
+
+babel
+  .transformAsync("REPLACE_ME;", {
+    configFile: false,
+    plugins: [(await import("./plugins/eager.cjs")).default],
+  })
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/eager-plugin-required.cjs

@@ -0,0 +1,14 @@
+/*
+ * This test throws an error, because the plugin accesses
+ * @babel/core's CJS .types export before that the ESM
+ * version is loaded.
+ */
+
+const babel = require("../../../cjs-proxy.cjs");
+
+babel
+  .transformAsync("REPLACE_ME;", {
+    configFile: false,
+    plugins: [require("./plugins/eager.cjs")],
+  })
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/lazy-plugin-as-string.cjs

@@ -0,0 +1,8 @@
+const babel = require("../../../cjs-proxy.cjs");
+
+babel
+  .transformAsync("REPLACE_ME;", {
+    configFile: false,
+    plugins: [__dirname + "/plugins/lazy.cjs"],
+  })
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/lazy-plugin-required.cjs

@@ -0,0 +1,8 @@
+const babel = require("../../../cjs-proxy.cjs");
+
+babel
+  .transformAsync("REPLACE_ME;", {
+    configFile: false,
+    plugins: [require("./plugins/lazy.cjs")],
+  })
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/plugins/eager.cjs

@@ -0,0 +1,13 @@
+const { types: t } = require("../../../../cjs-proxy.cjs");
+
+module.exports = function () {
+  return {
+    visitor: {
+      Identifier(path) {
+        if (path.node.name === "REPLACE_ME") {
+          path.replaceWith(t.stringLiteral("Replaced!"));
+        }
+      },
+    },
+  };
+};

packages/babel-core/test/fixtures/esm-cjs-integration/plugins/lazy.cjs

@@ -0,0 +1,34 @@
+/*
+import { types as t } from "../../../../cjs-proxy.cjs";
+
+export default function () {
+  return {
+    visitor: {
+      Identifier(path) {
+        if (path.node.name === "REPLACE_ME") {
+          path.replaceWith(t.stringLiteral("Replaced!"));
+        }
+      }
+    }
+  }
+}
+*/
+
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true,
+});
+exports["default"] = _default;
+var _core = require("../../../../cjs-proxy.cjs");
+function _default() {
+  return {
+    visitor: {
+      Identifier: function Identifier(path) {
+        if (path.node.name === "REPLACE_ME") {
+          path.replaceWith(_core.types.stringLiteral("Replaced!"));
+        }
+      },
+    },
+  };
+}

packages/babel-core/test/fixtures/esm-cjs-integration/transform-sync-after-dynamic-esm-import.cjs

@@ -0,0 +1,3 @@
+import("../../../lib/index.js")
+  .then(babel => babel.transformSync("REPLACE_ME;", { configFile: false }))
+  .then(out => console.log(out.code), console.error);

packages/babel-core/test/fixtures/esm-cjs-integration/transform-sync-after-static-esm-import.mjs

@@ -0,0 +1,4 @@
+import * as babel from "../../../lib/index.js";
+
+const out = babel.transformSync("REPLACE_ME;", { configFile: false });
+console.log(out.code);

packages/babel-core/test/fixtures/esm-cjs-integration/transform-sync.cjs

@@ -0,0 +1,9 @@
+/*
+ * This test throws an error, because the CJS .transformSync
+ * is called before that the ESM version is loaded.
+ */
+
+const babel = require("../../../cjs-proxy.cjs");
+
+const out = babel.transformSync("REPLACE_ME;", { configFile: false });
+console.log(out.code);

packages/babel-core/test/helpers/esm.js

@@ -1,6 +1,7 @@
 import cp from "child_process";
 import util from "util";
 import path from "path";
+import fs from "fs";
 import { fileURLToPath } from "url";
 import { createRequire } from "module";
 
@@ -12,6 +13,19 @@ const dirname = path.dirname(fileURLToPath(import.meta.url));
 // "minNodeVersion": "10.0.0" <-- For Ctrl+F when dropping node 10
 export const supportsESM = parseInt(process.versions.node) >= 12;
 
+export const outputType = (() => {
+  try {
+    return fs
+      .readFileSync(
+        new URL("../../../../.module-type", import.meta.url),
+        "utf-8",
+      )
+      .trim();
+  } catch (_) {
+    return "script";
+  }
+})();
+
 export const isMJS = file => path.extname(file) === ".mjs";
 
 export const itESM = supportsESM ? it : it.skip;