Expose caller.supportsStaticESM as a flag to change preset-env behavior.

packages/babel-preset-env/src/index.js

@@ -155,6 +155,10 @@ const filterItems = (
   return result;
 };
 
+function supportsStaticESM(caller) {
+  return !!(caller && caller.supportsStaticESM);
+}
+
 export default declare((api, opts) => {
   api.assertVersion(7);
 
@@ -232,9 +236,15 @@ export default declare((api, opts) => {
   const plugins = [];
   const pluginUseBuiltIns = useBuiltIns !== false;
 
+  if (
+    modules !== false &&
+    moduleTransformations[modules] &&
+    // TODO: Remove the 'api.caller' check eventually. Just here to prevent
+    // unnecessary breakage in the short term for users on older betas/RCs.
+    (modules !== "auto" || !api.caller || !api.caller(supportsStaticESM))
+  ) {
     // NOTE: not giving spec here yet to avoid compatibility issues when
     // transform-modules-commonjs gets its spec mode
-  if (modules !== false && moduleTransformations[modules]) {
     plugins.push([getPlugin(moduleTransformations[modules]), { loose }]);
   }
 

packages/babel-preset-env/src/module-transformations.js

@@ -1,4 +1,5 @@
 export default {
+  auto: "transform-modules-commonjs",
   amd: "transform-modules-amd",
   commonjs: "transform-modules-commonjs",
   cjs: "transform-modules-commonjs",

packages/babel-preset-env/src/normalize-options.js

@@ -128,13 +128,16 @@ export const validateIgnoreBrowserslistConfig = (
   );
 
 export const validateModulesOption = (
-  modulesOpt: ModuleOption = ModulesOption.commonjs,
+  modulesOpt: ModuleOption = ModulesOption.auto,
 ) => {
   invariant(
     ModulesOption[modulesOpt] ||
       ModulesOption[modulesOpt] === ModulesOption.false,
-    `Invalid Option: The 'modules' option must be either 'false' to indicate no modules, or a
-    module type which can be be one of: 'commonjs' (default), 'amd', 'umd', 'systemjs'.`,
+    `Invalid Option: The 'modules' option must be one of \n` +
+      ` - 'false' to indicate no module processing\n` +
+      ` - a specific module type: 'commonjs', 'amd', 'umd', 'systemjs'` +
+      ` - 'auto' (default) which will automatically select 'false' if the current\n` +
+      `   process is known to support ES module syntax, or "commonjs" otherwise\n`,
   );
 
   return modulesOpt;

packages/babel-preset-env/src/options.js

@@ -15,6 +15,7 @@ export const TopLevelOptions = {
 
 export const ModulesOption = {
   false: false,
+  auto: "auto",
   amd: "amd",
   commonjs: "commonjs",
   cjs: "cjs",

packages/babel-preset-env/test/debug-fixtures/android/stdout.txt

@@ -5,7 +5,7 @@ Using targets:
   "android": "4"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "android":"4" }

packages/babel-preset-env/test/debug-fixtures/builtins-no-import/stdout.txt

@@ -5,7 +5,7 @@ Using targets:
   "node": "6"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-function-name { "node":"6" }

packages/babel-preset-env/test/debug-fixtures/builtins/stdout.txt

@@ -7,7 +7,7 @@ Using targets:
   "node": "6"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"10" }

packages/babel-preset-env/test/debug-fixtures/electron/stdout.txt

@@ -12,7 +12,7 @@ Using targets:
   "electron": "0.36"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-function-name { "electron":"0.36" }

packages/babel-preset-env/test/debug-fixtures/plugins-only/stdout.txt

@@ -13,7 +13,7 @@ Using targets:
   "node": "7.4"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-literals { "firefox":"52" }

packages/babel-preset-env/test/debug-fixtures/shippedProposals-chrome60/stdout.txt

@@ -5,7 +5,7 @@ Using targets:
   "chrome": "60"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-dotall-regex { "chrome":"60" }

packages/babel-preset-env/test/debug-fixtures/shippedProposals/stdout.txt

@@ -3,7 +3,7 @@
 Using targets:
 {}
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals {}

packages/babel-preset-env/test/debug-fixtures/specific-targets/stdout.txt

@@ -10,7 +10,7 @@ Using targets:
   "safari": "7"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"10", "safari":"7" }

packages/babel-preset-env/test/debug-fixtures/usage-none/stdout.txt

@@ -7,7 +7,7 @@ Using targets:
   "ie": "11"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"11" }

packages/babel-preset-env/test/debug-fixtures/usage-with-import/stdout.txt

@@ -5,7 +5,7 @@ Using targets:
   "chrome": "55"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-dotall-regex { "chrome":"55" }

packages/babel-preset-env/test/debug-fixtures/usage/stdout.txt

@@ -7,7 +7,7 @@ Using targets:
   "ie": "11"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"11" }

packages/babel-preset-env/test/debug-fixtures/versions-decimals/stdout.txt

@@ -16,7 +16,7 @@ Using targets:
   "node": "6.1"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"10" }

packages/babel-preset-env/test/debug-fixtures/versions-strings/stdout.txt

@@ -7,7 +7,7 @@ Using targets:
   "node": "6.10"
 }
 
-Using modules transform: commonjs
+Using modules transform: auto
 
 Using plugins:
   transform-template-literals { "ie":"10" }

packages/babel-preset-env/test/fixtures/modules/auto-cjs/input.mjs

@@ -0,0 +1,2 @@
+import mod from "mod";
+console.log(mod);

packages/babel-preset-env/test/fixtures/modules/auto-cjs/options.json

@@ -0,0 +1,9 @@
+{
+  "caller": {
+    "name": "test-fixture",
+    "supportsStaticESM": false
+  },
+  "presets": [
+    "env"
+  ]
+}

packages/babel-preset-env/test/fixtures/modules/auto-cjs/output.js

@@ -0,0 +1,7 @@
+"use strict";
+
+var _mod = _interopRequireDefault(require("mod"));
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+console.log(_mod.default);

packages/babel-preset-env/test/fixtures/modules/auto-esm/input.mjs

@@ -0,0 +1,2 @@
+import mod from "mod";
+console.log(mod);

packages/babel-preset-env/test/fixtures/modules/auto-esm/options.json

@@ -0,0 +1,9 @@
+{
+  "caller": {
+    "name": "test-fixture",
+    "supportsStaticESM": true
+  },
+  "presets": [
+    "env"
+  ]
+}

packages/babel-preset-env/test/fixtures/modules/auto-esm/output.mjs

@@ -0,0 +1,2 @@
+import mod from "mod";
+console.log(mod);

packages/babel-preset-env/test/fixtures/modules/auto-unknown/input.mjs

@@ -0,0 +1,2 @@
+import mod from "mod";
+console.log(mod);

packages/babel-preset-env/test/fixtures/modules/auto-unknown/options.json

@@ -0,0 +1,5 @@
+{
+  "presets": [
+    "env"
+  ]
+}

packages/babel-preset-env/test/fixtures/modules/auto-unknown/output.js

@@ -0,0 +1,7 @@
+"use strict";
+
+var _mod = _interopRequireDefault(require("mod"));
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+console.log(_mod.default);

packages/babel-preset-env/test/normalize-options.spec.js

@@ -134,16 +134,20 @@ describe("normalize-options", () => {
   });
 
   describe("validateModulesOption", () => {
-    it("`undefined` option returns commonjs", () => {
-      expect(validateModulesOption()).toBe("commonjs");
+    it("`undefined` option returns auto", () => {
+      expect(validateModulesOption()).toBe("auto");
     });
 
-    it("`false` option returns commonjs", () => {
+    it("`false` option returns false", () => {
       expect(validateModulesOption(false)).toBe(false);
     });
 
+    it("auto option is valid", () => {
+      expect(validateModulesOption("auto")).toBe("auto");
+    });
+
     it("commonjs option is valid", () => {
-      expect(validateModulesOption()).toBe("commonjs");
+      expect(validateModulesOption("commonjs")).toBe("commonjs");
     });
 
     it("systemjs option is valid", () => {