module: warn on detection in typeless package

lib/internal/modules/esm/get_format.js

@@ -5,6 +5,7 @@ const {
   ObjectPrototypeHasOwnProperty,
   PromisePrototypeThen,
   PromiseResolve,
+  SafeSet,
   StringPrototypeIncludes,
   StringPrototypeCharCodeAt,
   StringPrototypeSlice,
@@ -19,7 +20,7 @@ const {
 const experimentalNetworkImports =
   getOptionValue('--experimental-network-imports');
 const { containsModuleSyntax } = internalBinding('contextify');
-const { getPackageType } = require('internal/modules/package_json_reader');
+const { getPackageScopeConfig, getPackageType } = require('internal/modules/package_json_reader');
 const { fileURLToPath } = require('internal/url');
 const { ERR_UNKNOWN_FILE_EXTENSION } = require('internal/errors').codes;
 
@@ -81,6 +82,7 @@ function underNodeModules(url) {
   return StringPrototypeIncludes(url.pathname, '/node_modules/');
 }
 
+let typelessPackageJsonFilesWarnedAbout;
 /**
  * @param {URL} url
  * @param {{parentURL: string; source?: Buffer}} context
@@ -92,7 +94,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
   const ext = extname(url);
 
   if (ext === '.js') {
-    const packageType = getPackageType(url);
+    const { type: packageType, pjsonPath } = getPackageScopeConfig(url);
     if (packageType !== 'none') {
       return packageType;
     }
@@ -111,9 +113,23 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
         // `source` is undefined when this is called from `defaultResolve`;
         // but this gets called again from `defaultLoad`/`defaultLoadSync`.
         if (getOptionValue('--experimental-detect-module')) {
-          return source ?
+          const format = source ?
             (containsModuleSyntax(`${source}`, fileURLToPath(url)) ? 'module' : 'commonjs') :
             null;
+          if (format === 'module') {
+            // This module has a .js extension, a package.json with no `type` field, and ESM syntax.
+            // Warn about the missing `type` field so that the user can avoid the performance penalty of detection.
+            typelessPackageJsonFilesWarnedAbout ??= new SafeSet();
+            if (!typelessPackageJsonFilesWarnedAbout.has(pjsonPath)) {
+              const warning = `${url} parsed as an ES module because module syntax was detected;` +
+              ` to avoid the performance penalty of syntax detection, add "type": "module" to ${pjsonPath}`;
+              process.emitWarning(warning, {
+                code: 'MODULE_TYPELESS_PACKAGE_JSON',
+              });
+              typelessPackageJsonFilesWarnedAbout.add(pjsonPath);
+            }
+          }
+          return format;
         }
         return 'commonjs';
       }

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

@@ -101,6 +101,7 @@ describe('--experimental-detect-module', { concurrency: true }, () => {
     ]) {
       it(testName, async () => {
         const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+          '--no-warnings',
           '--experimental-detect-module',
           entryPath,
         ]);
@@ -142,6 +143,7 @@ describe('--experimental-detect-module', { concurrency: true }, () => {
     ]) {
       it(testName, async () => {
         const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+          '--no-warnings',
           '--experimental-detect-module',
           entryPath,
         ]);
@@ -291,6 +293,7 @@ describe('--experimental-detect-module', { concurrency: true }, () => {
 
     it('permits declaration of CommonJS module variables', async () => {
       const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+        '--no-warnings',
         '--experimental-detect-module',
         fixtures.path('es-modules/package-without-type/commonjs-wrapper-variables.js'),
       ]);
@@ -327,6 +330,48 @@ describe('--experimental-detect-module', { concurrency: true }, () => {
       strictEqual(signal, null);
     });
   });
+
+  describe('warn about typeless packages for .js files with ESM syntax', { concurrency: true }, () => {
+    for (const { testName, entryPath } of [
+      {
+        testName: 'warns for ESM syntax in a .js entry point in a typeless package',
+        entryPath: fixtures.path('es-modules/package-without-type/module.js'),
+      },
+      {
+        testName: 'warns for ESM syntax in a .js file imported by a CommonJS entry point in a typeless package',
+        entryPath: fixtures.path('es-modules/package-without-type/imports-esm.js'),
+      },
+      {
+        testName: 'warns for ESM syntax in a .js file imported by an ESM entry point in a typeless package',
+        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,
+        ]);
+
+        match(stderr, /MODULE_TYPELESS_PACKAGE_JSON/);
+        strictEqual(stdout, 'executed\n');
+        strictEqual(code, 0);
+        strictEqual(signal, null);
+      });
+    }
+
+    it('warns only once for a package.json that affects multiple files', async () => {
+      const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+        '--experimental-detect-module',
+        fixtures.path('es-modules/package-without-type/detected-as-esm.js'),
+      ]);
+
+      match(stderr, /MODULE_TYPELESS_PACKAGE_JSON/);
+      strictEqual(stderr.match(/MODULE_TYPELESS_PACKAGE_JSON/g).length, 1);
+      strictEqual(stdout, 'executed\nexecuted\n');
+      strictEqual(code, 0);
+      strictEqual(signal, null);
+    });
+  });
 });
 
 // Validate temporarily disabling `--abort-on-uncaught-exception`

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

@@ -0,0 +1,2 @@
+import './module.js';
+console.log('executed');