Add tests

test/es-module/test-esm-detect-ambiguous.mjs

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+import { spawnPromisified } from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import { spawn } from 'node:child_process';
+import { describe, it } from 'node:test';
+import { strictEqual, match } from 'node:assert';
+
+describe('--experimental-detect-module', { concurrency: true }, () => {
+  describe('string input', { concurrency: true }, () => {
+    it('permits ESM syntax in --eval input without requiring --input-type=module', async () => {
+      const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+        '--experimental-detect-module',
+        '--eval',
+        'import { version } from "node:process"; console.log(version);',
+      ]);
+
+      strictEqual(stderr, '');
+      strictEqual(stdout, `${process.version}\n`);
+      strictEqual(code, 0);
+      strictEqual(signal, null);
+    });
+
+    // ESM is unsupported for --print via --input-type=module
+
+    it('permits ESM syntax in STDIN input without requiring --input-type=module', async () => {
+      const child = spawn(process.execPath, [
+        '--experimental-detect-module',
+      ]);
+      child.stdin.end('console.log(typeof import.meta.resolve)');
+
+      match((await child.stdout.toArray()).toString(), /^function\r?\n$/);
+    });
+
+    it('should be overridden by --input-type', async () => {
+      const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
+        '--experimental-detect-module',
+        '--input-type=commonjs',
+        '--eval',
+        'import.meta.url',
+      ]);
+
+      match(stderr, /SyntaxError: Cannot use 'import\.meta' outside a module/);
+      strictEqual(stdout, '');
+      strictEqual(code, 1);
+      strictEqual(signal, null);
+    });
+
+    it('should be overridden by --experimental-default-type', async () => {
+      const { code, signal, stdout, stderr } = await spawnPromisified(process.execPath, [
+        '--experimental-detect-module',
+        '--experimental-default-type=commonjs',
+        '--eval',
+        'import.meta.url',
+      ]);
+
+      match(stderr, /SyntaxError: Cannot use 'import\.meta' outside a module/);
+      strictEqual(stdout, '');
+      strictEqual(code, 1);
+      strictEqual(signal, null);
+    });
+  });
+
+  describe('file input in a typeless package', { concurrency: true }, () => {
+    for (const { testName, entryPath } of [
+      {
+        testName: 'permits CommonJS syntax in a .js entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/commonjs.js'),
+      },
+      {
+        testName: 'permits ESM syntax in a .js entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/module.js'),
+      },
+      {
+        testName: 'permits CommonJS syntax in a .js file imported by a CommonJS entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/imports-commonjs.cjs'),
+      },
+      {
+        testName: 'permits ESM syntax in a .js file imported by a CommonJS entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/imports-esm.js'),
+      },
+      {
+        testName: 'permits CommonJS syntax in a .js file imported by an ESM entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/imports-commonjs.mjs'),
+      },
+      {
+        testName: 'permits ESM syntax in a .js file imported by an ESM entry point',
+        entryPath: fixtures.path('es-modules/package-without-type/imports-esm.mjs'),
+      },
+    ]) {
+      it(testName, async () => {
+        const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+          '--experimental-detect-module',
+          entryPath,
+        ]);
+
+        strictEqual(stderr, '');
+        strictEqual(stdout, 'executed\n');
+        strictEqual(code, 0);
+        strictEqual(signal, null);
+      });
+    }
+  });
+
+  describe('file input in a "type": "commonjs" package', { concurrency: true }, () => {
+    for (const { testName, entryPath } of [
+      {
+        testName: 'disallows ESM syntax in a .js entry point',
+        entryPath: fixtures.path('es-modules/package-type-commonjs/module.js'),
+      },
+      {
+        testName: 'disallows ESM syntax in a .js file imported by a CommonJS entry point',
+        entryPath: fixtures.path('es-modules/package-type-commonjs/imports-esm.js'),
+      },
+      {
+        testName: 'disallows ESM syntax in a .js file imported by an ESM entry point',
+        entryPath: fixtures.path('es-modules/package-type-commonjs/imports-esm.mjs'),
+      },
+    ]) {
+      it(testName, async () => {
+        const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+          '--experimental-detect-module',
+          entryPath,
+        ]);
+
+        match(stderr, /SyntaxError: Unexpected token 'export'/);
+        strictEqual(stdout, '');
+        strictEqual(code, 1);
+        strictEqual(signal, null);
+      });
+    }
+  });
+
+  describe('file input in a "type": "module" package', { concurrency: true }, () => {
+    for (const { testName, entryPath } of [
+      {
+        testName: 'disallows CommonJS syntax in a .js entry point',
+        entryPath: fixtures.path('es-modules/package-type-module/cjs.js'),
+      },
+      {
+        testName: 'disallows CommonJS syntax in a .js file imported by a CommonJS entry point',
+        entryPath: fixtures.path('es-modules/package-type-module/imports-commonjs.cjs'),
+      },
+      {
+        testName: 'disallows CommonJS syntax in a .js file imported by an ESM entry point',
+        entryPath: fixtures.path('es-modules/package-type-module/imports-commonjs.mjs'),
+      },
+    ]) {
+      it(testName, async () => {
+        const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+          '--experimental-detect-module',
+          entryPath,
+        ]);
+
+        match(stderr, /ReferenceError: module is not defined in ES module scope/);
+        strictEqual(stdout, '');
+        strictEqual(code, 1);
+        strictEqual(signal, null);
+      });
+    }
+  });
+});

test/fixtures/es-modules/package-type-commonjs/imports-esm.js

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+import('./module.js');

test/fixtures/es-modules/package-type-commonjs/imports-esm.mjs

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+import './module.js';

test/fixtures/es-modules/package-type-commonjs/module.js

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+export default 'module';
+console.log('executed');

test/fixtures/es-modules/package-type-module/imports-commonjs.cjs

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+import('./cjs.js');

test/fixtures/es-modules/package-type-module/imports-commonjs.mjs

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+import './cjs.js';

test/fixtures/es-modules/package-without-type/commonjs.js

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+module.exports = 'cjs';
+console.log('executed');

test/fixtures/es-modules/package-without-type/imports-commonjs.cjs

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+import('./commonjs.js');

test/fixtures/es-modules/package-without-type/imports-commonjs.mjs

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+import './commonjs.js';

test/fixtures/es-modules/package-without-type/imports-esm.js

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+import('./module.js');

test/fixtures/es-modules/package-without-type/imports-esm.mjs

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+import './module.js';

test/fixtures/es-modules/package-without-type/module.js

@@ -1,3 +1,2 @@
-// This file can be run or imported only if `--experimental-default-type=module` is set.
 export default 'module';
 console.log('executed');