module: do not warn for typeless package.json when there isn't one

It was intended that warnings should only be emitted for an existing package.json without a type. This fixes a confusing warning telling users to update /package.json when there are no package.json on the lookup path at all, like this: [MODULE_TYPELESS_PACKAGE_JSON] Warning: ... parsed as an ES module because module syntax was detected; to avoid the performance penalty of syntax detection, add "type": "module" to /package.json Drive-by: update the warning message to be clear about reparsing and make it clear what's actionable. PR-URL: #54045 Backport-PR-URL: #56927 Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com> Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com> Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com> Reviewed-By: Matteo Collina <matteo.collina@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Refs: #52697

Author: joyeecheung

lib/internal/modules/esm/get_format.js

@@ -95,6 +95,19 @@ function underNodeModules(url) {
 }
 
 let typelessPackageJsonFilesWarnedAbout;
+function warnTypelessPackageJsonFile(pjsonPath, url) {
+ typelessPackageJsonFilesWarnedAbout ??= new SafeSet();
+ if (!typelessPackageJsonFilesWarnedAbout.has(pjsonPath)) {
+ const warning = `Module type of ${url} is not specified and it doesn't parse as CommonJS.\n` +
+ 'Reparsing as ES module because module syntax was detected. This incurs a performance overhead.\n' +
+ `To eliminate this warning, add "type": "module" to ${pjsonPath}.`;
+ process.emitWarning(warning, {
+ code: 'MODULE_TYPELESS_PACKAGE_JSON',
+ });
+ typelessPackageJsonFilesWarnedAbout.add(pjsonPath);
+ }
+}
+
 /**
  * @param {URL} url
  * @param {{parentURL: string; source?: Buffer}} context
@@ -106,7 +119,7 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
  const ext = extname(url);
 
  if (ext === '.js') {
- const { type: packageType, pjsonPath } = getPackageScopeConfig(url);
+ const { type: packageType, pjsonPath, exists: foundPackageJson } = getPackageScopeConfig(url);
  if (packageType !== 'none') {
  return packageType;
  }
@@ -127,18 +140,10 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
  // For ambiguous files (no type field, .js extension) we return
  // undefined from `resolve` and re-run the check in `load`.
  const format = detectModuleFormat(source, url);
- if (format === 'module') {
+ if (format === 'module' && foundPackageJson) {
  // 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);
- }
+ warnTypelessPackageJsonFile(pjsonPath, url);
  }
  return format;
  }

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

@@ -352,6 +352,18 @@ describe('Module syntax detection', { concurrency: !process.env.TEST_PARALLEL },
  });
  }
 
+ it('does not warn when there are no package.json', async () => {
+ const { stdout, stderr, code, signal } = await spawnPromisified(process.execPath, [
+ fixtures.path('es-modules/loose.js'),
+ ]);
+
+ strictEqual(stderr, '');
+ 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, [
  fixtures.path('es-modules/package-without-type/detected-as-esm.js'),