esm: deprecate legacy main lookup for modules

PR-URL: #36918
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

Author: guybedford

doc/api/deprecations.md

@@ -2699,10 +2699,28 @@ resolutions not in `node_modules`. This means there will not be deprecation
 warnings for `"exports"` in dependencies. With `--pending-deprecation`, a
 runtime warning results no matter where the `"exports"` usage occurs.
 
+### DEP0XXX: Main index lookup and extension searching
+<!-- YAML
+changes:
+ - version: REPLACEME
+ pr-url: https://github.com/nodejs/node/pull/36918
+ description: Documentation-only deprecation
+ with `--pending-deprecation` support.
+-->
+
+Type: Documentation-only (supports [`--pending-deprecation`][])
+
+Previously, `index.js` and extension searching lookups would apply to
+`import 'pkg'` main entry point resolution, even when resolving ES modules.
+
+With this deprecation, all ES module main entry point resolutions require
+an explicit [`"exports"` or `"main"` entry][] with the exact file extension.
+
 [Legacy URL API]: url.md#url_legacy_url_api
 [NIST SP 800-38D]: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
 [RFC 6066]: https://tools.ietf.org/html/rfc6066#section-3
 [WHATWG URL API]: url.md#url_the_whatwg_url_api
+[`"exports"` or `"main"` entry]: packages.md#packages_main_entry_point_export
 [`--pending-deprecation`]: cli.md#cli_pending_deprecation
 [`--throw-deprecation`]: cli.md#cli_throw_deprecation
 [`--unhandled-rejections`]: cli.md#cli_unhandled_rejections_mode
@@ -2823,7 +2841,7 @@ runtime warning results no matter where the `"exports"` usage occurs.
 [from_string_encoding]: buffer.md#buffer_static_method_buffer_from_string_encoding
 [legacy `urlObject`]: url.md#url_legacy_urlobject
 [static methods of `crypto.Certificate()`]: crypto.md#crypto_class_certificate
-[subpath exports]: #packages_subpath_exports
-[subpath folder mappings]: #packages_subpath_folder_mappings
-[subpath imports]: #packages_subpath_imports
-[subpath patterns]: #packages_subpath_patterns
+[subpath exports]: packages.md#packages_subpath_exports
+[subpath folder mappings]: packages.md#packages_subpath_folder_mappings
+[subpath imports]: packages.md#packages_subpath_imports
+[subpath patterns]: packages.md#packages_subpath_patterns

lib/internal/modules/esm/resolve.js

@@ -90,6 +90,36 @@ function emitFolderMapDeprecation(match, pjsonUrl, isExports, base) {
 );
 }
 
+functionemitLegacyIndexDeprecation(url,packageJSONUrl,base,main){
+if(!pendingDeprecation)
+return;
+const{ format }=defaultGetFormat(url);
+if(format!=='module')
+return;
+constpath=fileURLToPath(url);
+constpkgPath=fileURLToPath(newURL('.',packageJSONUrl));
+constbasePath=fileURLToPath(base);
+if(main)
+process.emitWarning(
+`Package ${pkgPath} has a "main" field set to ${JSONStringify(main)}, `+
+`excluding the full filename and extension to the resolved file at "${
+StringPrototypeSlice(path,pkgPath.length)}", imported from ${
+basePath}.\n Automatic extension resolution of the "main" field is`+
+'deprecated for ES modules.',
+'DeprecationWarning',
+'DEP0150'
+);
+else
+process.emitWarning(
+`No "main" or "exports" field defined in the package.json for ${pkgPath
+} resolving the main entry point "${
+StringPrototypeSlice(path,pkgPath.length)}", imported from ${basePath
+}.\nDefault "index" lookups for the main are deprecated for ES modules.`,
+'DeprecationWarning',
+'DEP0150'
+);
+}
+
 functiongetConditionsSet(conditions){
 if(conditions!==undefined&&conditions!==DEFAULT_CONDITIONS){
 if(!ArrayIsArray(conditions)){
@@ -209,41 +239,33 @@ function legacyMainResolve(packageJSONUrl, packageConfig, base) {
 if(fileExists(guess=newURL(`./${packageConfig.main}`,
 packageJSONUrl))){
 returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}.js`,
-packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}.json`,
-packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}.node`,
-packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}/index.js`,
-packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}/index.json`,
-packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL(`./${packageConfig.main}/index.node`,
-packageJSONUrl))){
+}elseif(fileExists(guess=newURL(`./${packageConfig.main}.js`,
+packageJSONUrl)));
+elseif(fileExists(guess=newURL(`./${packageConfig.main}.json`,
+packageJSONUrl)));
+elseif(fileExists(guess=newURL(`./${packageConfig.main}.node`,
+packageJSONUrl)));
+elseif(fileExists(guess=newURL(`./${packageConfig.main}/index.js`,
+packageJSONUrl)));
+elseif(fileExists(guess=newURL(`./${packageConfig.main}/index.json`,
+packageJSONUrl)));
+elseif(fileExists(guess=newURL(`./${packageConfig.main}/index.node`,
+packageJSONUrl)));
+elseguess=undefined;
+if(guess){
+emitLegacyIndexDeprecation(guess,packageJSONUrl,base,
+packageConfig.main);
 returnguess;
 }
 // Fallthrough.
 }
-if(fileExists(guess=newURL('./index.js',packageJSONUrl))){
-returnguess;
-}
+if(fileExists(guess=newURL('./index.js',packageJSONUrl)));
 // So fs.
-if(fileExists(guess=newURL('./index.json',packageJSONUrl))){
-returnguess;
-}
-if(fileExists(guess=newURL('./index.node',packageJSONUrl))){
+elseif(fileExists(guess=newURL('./index.json',packageJSONUrl)));
+elseif(fileExists(guess=newURL('./index.node',packageJSONUrl)));
+elseguess=undefined;
+if(guess){
+emitLegacyIndexDeprecation(guess,packageJSONUrl,base,packageConfig.main);
 returnguess;
 }
 // Not found.
@@ -891,3 +913,6 @@ module.exports = {
  packageExportsResolve,
  packageImportsResolve
 };
+
+// cycle
+const{ defaultGetFormat }=require('internal/modules/esm/get_format');

test/es-module/test-esm-exports-pending-deprecations.mjs

@@ -6,7 +6,9 @@ let curWarning = 0;
 constexpectedWarnings=[
 '"./sub/"',
 '"./fallbackdir/"',
-'"./subpath/"'
+'"./subpath/"',
+'no_exports',
+'default_index'
 ];
 
 process.addListener('warning',mustCall((warning)=>{

test/es-module/test-esm-exports.mjs

@@ -35,7 +35,7 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
 ['pkgexports-sugar',{default: 'main'}],
 // Path patterns
 ['pkgexports/subpath/sub-dir1',{default: 'main'}],
-['pkgexports/features/dir1',{default: 'main'}]
+['pkgexports/features/dir1',{default: 'main'}],
 ]);
 
 if(isRequire){
@@ -44,6 +44,11 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
 validSpecifiers.set('pkgexports/subpath/dir1/',{default: 'main'});
 validSpecifiers.set('pkgexports/subpath/dir2',{default: 'index'});
 validSpecifiers.set('pkgexports/subpath/dir2/',{default: 'index'});
+}else{
+// No exports or main field
+validSpecifiers.set('no_exports',{default: 'index'});
+// Main field without extension
+validSpecifiers.set('default_index',{default: 'main'});
 }
 
 for(const[validSpecifier,expected]ofvalidSpecifiers){