module: CJS extension searching for folder exports

doc/api/modules.md

@@ -210,26 +210,32 @@ LOAD_SELF_REFERENCE(X, START)
 3. If the `package.json` has no "exports", return.
 4. If the name in `package.json` isn't a prefix of X, throw "not found".
 5. Otherwise, load the remainder of X relative to this package as if it
-   was loaded via `LOAD_NODE_MODULES` with a name in `package.json`.
+  was loaded via `LOAD_NODE_MODULES` with a name in `package.json`.
 
 LOAD_PACKAGE_EXPORTS(DIR, X)
 1. Try to interpret X as a combination of name and subpath where the name
    may have a @scope/ prefix and the subpath begins with a slash (`/`).
-2. If X matches this pattern and DIR/name/package.json is a file:
-   a. Parse DIR/name/package.json, and look for "exports" field.
-   b. If "exports" is null or undefined, return.
-   c. If "exports" is an object with some keys starting with "." and some keys
-      not starting with ".", throw "invalid config".
-   d. If "exports" is a string, or object with no keys starting with ".", treat
-      it as having that value as its "." object property.
-   e. If subpath is "." and "exports" does not have a "." entry, return.
-   f. Find the longest key in "exports" that the subpath starts with.
-   g. If no such key can be found, throw "not found".
-   h. let RESOLVED_URL =
-        PACKAGE_EXPORTS_TARGET_RESOLVE(pathToFileURL(DIR/name), exports[key],
-        subpath.slice(key.length), ["node", "require"]), as defined in the ESM
-        resolver.
-   i. Load fileURLToPath(RESOLVED_URL) as its file extension format.  STOP
+2. If X does not match this pattern or DIR/name/package.json is not a file,
+   return.
+3. Parse DIR/name/package.json, and look for "exports" field.
+4. If "exports" is null or undefined, return.
+5. If "exports" is an object with some keys starting with "." and some keys
+  not starting with ".", throw "invalid config".
+6. If "exports" is a string, or object with no keys starting with ".", treat
+  it as having that value as its "." object property.
+7. If subpath is "." and "exports" does not have a "." entry, return.
+8. Find the longest key in "exports" that the subpath starts with.
+9. If no such key can be found, throw "not found".
+10. let RESOLVED =
+    fileURLToPath(PACKAGE_EXPORTS_TARGET_RESOLVE(pathToFileURL(DIR/name),
+    exports[key], subpath.slice(key.length), ["node", "require"])), as defined
+    in the ESM resolver.
+11. If key ends with "/":
+    a. LOAD_AS_FILE(RESOLVED)
+    b. LOAD_AS_DIRECTORY(RESOLVED)
+12. Otherwise
+   a. If RESOLVED is a file, load it as its file extension format.  STOP
+13. Throw "not found"
 ```
 
 ## Caching

lib/internal/modules/cjs/loader.js

@@ -37,6 +37,7 @@ const {
   ObjectPrototypeHasOwnProperty,
   ObjectSetPrototypeOf,
   ReflectSet,
+  RegExpPrototypeTest,
   SafeMap,
   String,
   StringPrototypeIndexOf,
@@ -125,7 +126,7 @@ function enrichCJSError(err) {
     after a comment block and/or after a variable definition.
   */
   if (err.message.startsWith('Unexpected token \'export\'') ||
-    (/^\s*import(?=[ {'"*])\s*(?![ (])/).test(lineWithErr)) {
+    (RegExpPrototypeTest(/^\s*import(?=[ {'"*])\s*(?![ (])/, lineWithErr))) {
     // Emit the warning synchronously because we are in the middle of handling
     // a SyntaxError that will throw and likely terminate the process before an
     // asynchronous warning would be emitted.
@@ -352,10 +353,11 @@ const realpathCache = new Map();
 // absolute realpath.
 function tryFile(requestPath, isMain) {
   const rc = stat(requestPath);
+  if (rc !== 0) return;
   if (preserveSymlinks && !isMain) {
-    return rc === 0 && path.resolve(requestPath);
+    return path.resolve(requestPath);
   }
-  return rc === 0 && toRealPath(requestPath);
+  return toRealPath(requestPath);
 }
 
 function toRealPath(requestPath) {
@@ -410,7 +412,7 @@ function trySelf(parentPath, request) {
   if (fromExports) {
     return tryFile(fromExports, false);
   }
-  assert(false);
+  assert(fromExports !== false);
 }
 
 function isConditionalDotExportSugar(exports, basePath) {
@@ -466,8 +468,24 @@ function applyExports(basePath, expansion) {
     if (dirMatch !== '') {
       const mapping = pkgExports[dirMatch];
       const subpath = StringPrototypeSlice(mappingKey, dirMatch.length);
-      return resolveExportsTarget(pathToFileURL(basePath + '/'), mapping,
-                                  subpath, mappingKey);
+      const resolved = resolveExportsTarget(pathToFileURL(basePath + '/'),
+                                            mapping, subpath, mappingKey);
+      // Extension searching for folder exports only
+      const rc = stat(resolved);
+      if (rc === 0) return resolved;
+      if (!(RegExpPrototypeTest(trailingSlashRegex, resolved))) {
+        const exts = ObjectKeys(Module._extensions);
+        const filename = tryExtensions(resolved, exts, false);
+        if (filename) return filename;
+      }
+      if (rc === 1) {
+        const exts = ObjectKeys(Module._extensions);
+        const filename = tryPackage(resolved, exts, false,
+                                    basePath + expansion);
+        if (filename) return filename;
+      }
+      // Undefined means not found
+      return;
     }
   }
 
@@ -488,10 +506,10 @@ function resolveExports(nmPath, request) {
 
   const basePath = path.resolve(nmPath, name);
   const fromExports = applyExports(basePath, expansion);
-  if (!fromExports) {
-    return false;
+  if (fromExports) {
+    return tryFile(fromExports, false);
   }
-  return tryFile(fromExports, false);
+  return fromExports;
 }
 
 function isArrayIndex(p) {
@@ -582,6 +600,7 @@ function resolveExportsTarget(baseUrl, target, subpath, mappingKey) {
     StringPrototypeSlice(baseUrl.pathname, 0, -1), mappingKey, subpath, target);
 }
 
+const trailingSlashRegex = /(?:^|\/)\.?\.$/;
 Module._findPath = function(request, paths, isMain) {
   const absoluteRequest = path.isAbsolute(request);
   if (absoluteRequest) {
@@ -600,7 +619,7 @@ Module._findPath = function(request, paths, isMain) {
   let trailingSlash = request.length > 0 &&
     request.charCodeAt(request.length - 1) === CHAR_FORWARD_SLASH;
   if (!trailingSlash) {
-    trailingSlash = /(?:^|\/)\.?\.$/.test(request);
+    trailingSlash = RegExpPrototypeTest(trailingSlashRegex, request);
   }
 
   // For each path
@@ -611,6 +630,9 @@ Module._findPath = function(request, paths, isMain) {
 
     if (!absoluteRequest) {
       const exportsResolved = resolveExports(curPath, request);
+      // Undefined means not found, false means no exports
+      if (exportsResolved === undefined)
+        break;
       if (exportsResolved) {
         return exportsResolved;
       }

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

@@ -35,6 +35,14 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
     ['pkgexports-sugar', { default: 'main' }],
   ]);
 
+  if (isRequire) {
+    validSpecifiers.set('pkgexports/subpath/file', { default: 'file' });
+    validSpecifiers.set('pkgexports/subpath/dir1', { default: 'main' });
+    validSpecifiers.set('pkgexports/subpath/dir1/', { default: 'main' });
+    validSpecifiers.set('pkgexports/subpath/dir2', { default: 'index' });
+    validSpecifiers.set('pkgexports/subpath/dir2/', { default: 'index' });
+  }
+
   for (const [validSpecifier, expected] of validSpecifiers) {
     if (validSpecifier === null) continue;
 
@@ -118,23 +126,28 @@ import fromInside from '../fixtures/node_modules/pkgexports/lib/hole.js';
     }));
   }
 
-  // Non-existing file
-  loadFixture('pkgexports/sub/not-a-file.js').catch(mustCall((err) => {
-    strictEqual(err.code, (isRequire ? '' : 'ERR_') + 'MODULE_NOT_FOUND');
-    // ESM returns a full file path
-    assertStartsWith(err.message, isRequire ?
-      'Cannot find module \'pkgexports/sub/not-a-file.js\'' :
-      'Cannot find module');
-  }));
+  const notFoundExports = new Map([
+    // Non-existing file
+    ['pkgexports/sub/not-a-file.js', 'pkgexports/sub/not-a-file.js'],
+    // No extension lookups
+    ['pkgexports/no-ext', 'pkgexports/no-ext'],
+  ]);
 
-  // No extension lookups
-  loadFixture('pkgexports/no-ext').catch(mustCall((err) => {
-    strictEqual(err.code, (isRequire ? '' : 'ERR_') + 'MODULE_NOT_FOUND');
-    // ESM returns a full file path
-    assertStartsWith(err.message, isRequire ?
-      'Cannot find module \'pkgexports/no-ext\'' :
-      'Cannot find module');
-  }));
+  if (!isRequire) {
+    notFoundExports.set('pkgexports/subpath/file', 'pkgexports/subpath/file');
+    notFoundExports.set('pkgexports/subpath/dir1', 'pkgexports/subpath/dir1');
+    notFoundExports.set('pkgexports/subpath/dir2', 'pkgexports/subpath/dir2');
+  }
+
+  for (const [specifier, request] of notFoundExports) {
+    loadFixture(specifier).catch(mustCall((err) => {
+      strictEqual(err.code, (isRequire ? '' : 'ERR_') + 'MODULE_NOT_FOUND');
+      // ESM returns a full file path
+      assertStartsWith(err.message, isRequire ?
+        `Cannot find module '${request}'` :
+        'Cannot find module');
+    }));
+  }
 
   // The use of %2F escapes in paths fails loading
   loadFixture('pkgexports/sub/..%2F..%2Fbar.js').catch(mustCall((err) => {