module: package exports dot main support

doc/api/esm.md

@@ -270,6 +270,31 @@ If a package has no exports, setting `"exports": false` can be used instead of
 `"exports": {}` to indicate the package does not intend for submodules to be
 exposed.
 
+Exports can also be used to map the main entry point of a package:
+
+<!-- eslint-skip -->
+```js
+// ./node_modules/es-module-package/package.json
+{
+  "exports": {
+    ".": "./main.js"
+  }
+}
+```
+
+where the "." indicates loading the package without any subpath. Exports will
+always override any existing main for both CJS and ESM.
+
+For packages with only a main, exports as a string is also supported:
+
+<!-- eslint-skip -->
+```js
+// ./node_modules/es-module-package/package.json
+{
+  "exports": "./main.js"
+}
+```
+
 Any invalid exports entries will be ignored. This includes exports not
 starting with `"./"` or a missing trailing `"/"` for directory exports.
 
@@ -781,6 +806,15 @@ _isMain_ is **true** when resolving the Node.js application entry point.
 **PACKAGE_MAIN_RESOLVE**(_packageURL_, _pjson_)
 > 1. If _pjson_ is **null**, then
 >    1. Throw a _Module Not Found_ error.
+> 1. If _pjson.exports_ is not **null** or **undefined**, then
+>    1. If _pjson.exports_ is a String or Array, then
+>       1. Return _PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, pjson.exports,
+>          "")_.
+>    1. If _pjson.exports is an Object, then
+>       1. If _pjson.exports_ contains a _"."_ property, then
+>          1. Let _mainExport_ be the _"."_ property in _pjson.exports_.
+>          1. Return _PACKAGE_EXPORTS_TARGET_RESOLVE(packageURL, mainExport,
+>             "")_.
 > 1. If _pjson.main_ is a String, then
 >    1. Let _resolvedMain_ be the URL resolution of _packageURL_, "/", and
 >       _pjson.main_.

lib/internal/modules/cjs/loader.js

@@ -360,11 +360,11 @@ function findLongestRegisteredExtension(filename) {
 // This only applies to requests of a specific form:
 // 1. name/.*
 // 2. @scope/name/.*
-const EXPORTS_PATTERN = /^((?:@[^/\\%]+\/)?[^./\\%][^/\\%]*)(\/.*)$/;
+const EXPORTS_PATTERN = /^((?:@[^/\\%]+\/)?[^./\\%][^/\\%]*)(\/.*)?$/;
 function resolveExports(nmPath, request, absoluteRequest) {
   // The implementation's behavior is meant to mirror resolution in ESM.
   if (experimentalExports && !absoluteRequest) {
-    const [, name, expansion] =
+    const [, name, expansion = ''] =
       StringPrototype.match(request, EXPORTS_PATTERN) || [];
     if (!name) {
       return path.resolve(nmPath, request);
@@ -397,6 +397,10 @@ function resolveExports(nmPath, request, absoluteRequest) {
                                     subpath, basePath, mappingKey);
       }
     }
+    if (mappingKey === '.' && typeof pkgExports === 'string') {
+      return resolveExportsTarget(pathToFileURL(basePath + '/'), pkgExports,
+                                  '', basePath, mappingKey);
+    }
     if (pkgExports != null) {
       // eslint-disable-next-line no-restricted-syntax
       const e = new Error(`Package exports for '${basePath}' do not define ` +

src/module_wrap.cc

@@ -773,39 +773,6 @@ Maybe<URL> FinalizeResolution(Environment* env,
   return Just(resolved);
 }
 
-Maybe<URL> PackageMainResolve(Environment* env,
-                              const URL& pjson_url,
-                              const PackageConfig& pcfg,
-                              const URL& base) {
-  if (pcfg.exists == Exists::Yes) {
-    if (pcfg.has_main == HasMain::Yes) {
-      URL resolved(pcfg.main, pjson_url);
-      const std::string& path = resolved.ToFilePath();
-      if (CheckDescriptorAtPath(path) == FILE) {
-        return Just(resolved);
-      }
-    }
-    if (env->options()->es_module_specifier_resolution == "node") {
-      if (pcfg.has_main == HasMain::Yes) {
-        return FinalizeResolution(env, URL(pcfg.main, pjson_url), base);
-      } else {
-        return FinalizeResolution(env, URL("index", pjson_url), base);
-      }
-    }
-    if (pcfg.type != PackageType::Module) {
-      Maybe<URL> resolved = LegacyMainResolve(pjson_url, pcfg);
-      if (!resolved.IsNothing()) {
-        return resolved;
-      }
-    }
-  }
-  std::string msg = "Cannot find main entry point for " +
-      URL(".", pjson_url).ToFilePath() + " imported from " +
-      base.ToFilePath();
-  node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
-  return Nothing<URL>();
-}
-
 void ThrowExportsNotFound(Environment* env,
                           const std::string& subpath,
                           const URL& pjson_url,
@@ -887,6 +854,98 @@ Maybe<URL> ResolveExportsTarget(Environment* env,
   return Just(subpath_resolved);
 }
 
+Maybe<URL> PackageMainResolve(Environment* env,
+                              const URL& pjson_url,
+                              const PackageConfig& pcfg,
+                              const URL& base) {
+  if (pcfg.exists == Exists::Yes) {
+    Isolate* isolate = env->isolate();
+    Local<Context> context = env->context();
+    if (!pcfg.exports.IsEmpty()) {
+      Local<Value> exports = pcfg.exports.Get(isolate);
+      if (exports->IsString() || exports->IsObject() || exports->IsArray()) {
+        Local<Value> target;
+        if (!exports->IsObject()) {
+          target = exports;
+        } else {
+          Local<Object> exports_obj = exports.As<Object>();
+          Local<String> dot_string = String::NewFromUtf8(env->isolate(), ".",
+              v8::NewStringType::kNormal).ToLocalChecked();
+          target = exports_obj->Get(env->context(), dot_string).ToLocalChecked();
+        }
+        if (target->IsString()) {
+          Utf8Value target_utf8(isolate, target.As<v8::String>());
+          std::string target(*target_utf8, target_utf8.length());
+          Maybe<URL> resolved = ResolveExportsTarget(env, target, "", ".",
+              pjson_url, base);
+          if (resolved.IsNothing()) {
+            ThrowExportsInvalid(env, ".", target, pjson_url, base);
+            return Nothing<URL>();
+          }
+          return FinalizeResolution(env, resolved.FromJust(), base);
+        } else if (target->IsArray()) {
+          Local<Array> target_arr = target.As<Array>();
+          const uint32_t length = target_arr->Length();
+          if (length == 0) {
+            ThrowExportsInvalid(env, ".", target, pjson_url, base);
+            return Nothing<URL>();
+          }
+          for (uint32_t i = 0; i < length; i++) {
+            auto target_item = target_arr->Get(context, i).ToLocalChecked();
+            if (target_item->IsString()) {
+              Utf8Value target_utf8(isolate, target_item.As<v8::String>());
+              std::string target_str(*target_utf8, target_utf8.length());
+              Maybe<URL> resolved = ResolveExportsTarget(env, target_str, "",
+                  ".", pjson_url, base, false);
+              if (resolved.IsNothing()) continue;
+              return FinalizeResolution(env, resolved.FromJust(), base);
+            }
+          }
+          auto invalid = target_arr->Get(context, length - 1).ToLocalChecked();
+          if (!invalid->IsString()) {
+            ThrowExportsInvalid(env, ".", invalid, pjson_url, base);
+            return Nothing<URL>();
+          }
+          Utf8Value invalid_utf8(isolate, invalid.As<v8::String>());
+          std::string invalid_str(*invalid_utf8, invalid_utf8.length());
+          Maybe<URL> resolved = ResolveExportsTarget(env, invalid_str, "",
+                                                    ".", pjson_url, base);
+          CHECK(resolved.IsNothing());
+          return Nothing<URL>();
+        } else {
+          ThrowExportsInvalid(env, ".", target, pjson_url, base);
+          return Nothing<URL>();
+        }
+      }
+    }
+    if (pcfg.has_main == HasMain::Yes) {
+      URL resolved(pcfg.main, pjson_url);
+      const std::string& path = resolved.ToFilePath();
+      if (CheckDescriptorAtPath(path) == FILE) {
+        return Just(resolved);
+      }
+    }
+    if (env->options()->es_module_specifier_resolution == "node") {
+      if (pcfg.has_main == HasMain::Yes) {
+        return FinalizeResolution(env, URL(pcfg.main, pjson_url), base);
+      } else {
+        return FinalizeResolution(env, URL("index", pjson_url), base);
+      }
+    }
+    if (pcfg.type != PackageType::Module) {
+      Maybe<URL> resolved = LegacyMainResolve(pjson_url, pcfg);
+      if (!resolved.IsNothing()) {
+        return resolved;
+      }
+    }
+  }
+  std::string msg = "Cannot find main entry point for " +
+      URL(".", pjson_url).ToFilePath() + " imported from " +
+      base.ToFilePath();
+  node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
+  return Nothing<URL>();
+}
+
 Maybe<URL> PackageExportsResolve(Environment* env,
                                  const URL& pjson_url,
                                  const std::string& pkg_subpath,

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

@@ -20,6 +20,8 @@ import { requireFixture, importFixture } from '../fixtures/pkgexports.mjs';
     // Fallbacks
     ['pkgexports/fallbackdir/asdf.js', { default: 'asdf' }],
     ['pkgexports/fallbackfile', { default: 'asdf' }],
+    // Dot main
+    ['pkgexports', { default: 'asdf' }],
   ]);
   for (const [validSpecifier, expected] of validSpecifiers) {
     if (validSpecifier === null) continue;
@@ -81,18 +83,6 @@ import { requireFixture, importFixture } from '../fixtures/pkgexports.mjs';
     }));
   }
 
-  // There's no main field so we won't find anything when importing the name.
-  // The fact that "." is mapped is ignored, it's not a valid main config.
-  loadFixture('pkgexports').catch(mustCall((err) => {
-    if (isRequire) {
-      strictEqual(err.code, 'MODULE_NOT_FOUND');
-      assertStartsWith(err.message, 'Cannot find module \'pkgexports\'');
-    } else {
-      strictEqual(err.code, 'ERR_MODULE_NOT_FOUND');
-      assertStartsWith(err.message, 'Cannot find main entry point');
-    }
-  }));
-
   // Covering out bases - not a file is still not a file after dir mapping.
   loadFixture('pkgexports/sub/not-a-file.js').catch(mustCall((err) => {
     strictEqual(err.code, (isRequire ? '' : 'ERR_') + 'MODULE_NOT_FOUND');