fix: Respect package.json exports when adding extensions to package imports

.gitignore

@@ -11,3 +11,6 @@ dist/
 lerna-debug.log
 node_modules/
 npm-debug.log*
+
+# Fake packages used as test fixtures
+!src/test/fixtures/*/node_modules/

src/converter/convertFile.test.ts

@@ -39,6 +39,10 @@ describe('convertFile', () => {
       await testFileConversion('dynamic-imports');
     });
 
+    it('adds file extensions to package imports only when needed to resolve as ESM', async () => {
+      await testFileConversion('package-exports');
+    });
+
     it('handles index files referenced with a trailing slash', async () => {
       await testFileConversion('trailing-slash');
     });

src/test/fixtures/package-exports/src/main.snap.ts

@@ -0,0 +1,9 @@
+// Legacy package without "exports" gets file extensions
+import omit from 'legacy/omit.js';
+import dir from 'legacy/dir/index.js';
+// Package with "exports" stays untouched
+import sub from 'modern/sub';
+// Package root stays untouched
+import pkg from '@scope/pkg';
+
+console.log(omit, dir, sub, pkg);

src/test/fixtures/package-exports/src/main.ts

@@ -0,0 +1,9 @@
+// Legacy package without "exports" gets file extensions
+import omit from 'legacy/omit';
+import dir from 'legacy/dir';
+// Package with "exports" stays untouched
+import sub from 'modern/sub';
+// Package root stays untouched
+import pkg from '@scope/pkg';
+
+console.log(omit, dir, sub, pkg);

src/test/fixtures/package-exports/tsconfig.json

@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "module": "Node16",
+    "skipLibCheck": true,
+    "strict": true,
+    "target": "es2016"
+  },
+  "include": ["src"]
+}

src/util/PathUtil.ts

@@ -1,5 +1,4 @@
 import path from 'node:path';
-import fs from 'node:fs';
 import type {ModuleInfo} from '../parser/InfoParser.js';
 
 /**
@@ -55,10 +54,3 @@ export function hasRelativePath(path: string) {
   }
   return path.startsWith('./') || path.startsWith('../');
 }
-
-export function isNodeModuleRoot(directory: string): boolean {
-  const isInNodeModules = directory.includes(`node_modules${path.sep}`);
-  const packageJsonPath = path.join(directory, 'package.json');
-  const packageJsonExists = fs.existsSync(packageJsonPath);
-  return isInNodeModules && packageJsonExists;
-}

src/util/replaceModulePath.ts

@@ -1,8 +1,8 @@
-import {SourceFile, StringLiteral} from 'ts-morph';
+import {SourceFile, StringLiteral, ts} from 'ts-morph';
 import {ModuleInfo, parseInfo} from '../parser/InfoParser.js';
 import {ProjectUtil} from './ProjectUtil.js';
 import {toImport, toImportAttribute} from '../converter/ImportConverter.js';
-import {getNormalizedPath, isNodeModuleRoot} from './PathUtil.js';
+import {getNormalizedPath} from './PathUtil.js';
 import path from 'node:path';
 import {PathFinder} from './PathFinder.js';
 
@@ -49,24 +49,56 @@ function createReplacementPath({
       return toImportAttribute(info);
     }
 
+    if (isNodeModulesPath) {
+      return createPackageImportPath(info);
+    }
+
     // If an import does not have a file extension or isn't an extension recognized here and can't be found locally (perhaps
     // file had . in name), try to find a matching file by traversing through all valid TypeScript source file extensions.
-    let baseFilePath = comesFromPathAlias
+    const baseFilePath = comesFromPathAlias
       ? getNormalizedPath(projectDirectory, info, paths)
       : path.join(info.directory, info.normalized);
-    if (isNodeModulesPath) {
-      baseFilePath = path.join(projectDirectory, 'node_modules', info.normalized);
-    }
 
     const foundPath = PathFinder.findPath(baseFilePath, info.extension);
     if (foundPath) {
-      // TODO: Write test case for this condition, mock "path" and "fs" calls if necessary
-      if (foundPath.extension === '/index.js' && isNodeModuleRoot(baseFilePath)) {
-        // @fixes https://github.com/bennycode/ts2esm/issues/81#issuecomment-2437503011
-        return null;
-      }
       return toImport({...info, extension: foundPath.extension});
     }
   }
   return null;
 }
+
+const ESM_RESOLUTION_OPTIONS: ts.CompilerOptions = {
+  allowJs: true,
+  module: ts.ModuleKind.NodeNext,
+  moduleResolution: ts.ModuleResolutionKind.NodeNext,
+};
+
+// Resolves a specifier the way Node.js resolves it from an ES module, which respects "exports" in package.json
+function resolvesAsESM(specifier: string, containingFile: string) {
+  const {resolvedModule} = ts.resolveModuleName(
+    specifier,
+    containingFile,
+    ESM_RESOLUTION_OPTIONS,
+    ts.sys,
+    undefined,
+    undefined,
+    ts.ModuleKind.ESNext
+  );
+  return !!resolvedModule;
+}
+
+// Imports from packages only get an extension when they don't resolve as written but do with the extension.
+// This keeps imports of packages with "exports" (i.e. "firebase-functions/v1/https") untouched,
+// while legacy packages (i.e. "lodash/omit") become "lodash/omit.js".
+// @see https://github.com/bennycode/ts2esm/issues/128
+function createPackageImportPath(info: ModuleInfo) {
+  if (resolvesAsESM(info.normalized, info.sourceFilePath)) {
+    return null;
+  }
+  for (const extension of ['.js', '/index.js']) {
+    if (resolvesAsESM(`${info.normalized}${extension}`, info.sourceFilePath)) {
+      return toImport({...info, extension});
+    }
+  }
+  return null;
+}