fixup!: detect valid

Author: himself65

lib/internal/modules/esm/resolve.js

@@ -183,12 +183,6 @@ function legacyMainResolve(packageJSONUrl, packageConfig, base) {
 
   const baseStringified = isURL(base) ? base.href : base;
 
-  if (typeof packageConfig.main !== 'string') {
-    throw new ERR_INVALID_PACKAGE_CONFIG(
-      fileURLToPath(packageJSONUrl), base,
-      '"main" must be a string', 'main');
-  }
-
   const resolvedOption = FSLegacyMainResolve(packageJsonUrlString, packageConfig.main, baseStringified);
 
   const baseUrl = resolvedOption <= legacyMainResolveExtensionsIndexes.kResolvedByMainIndexNode ? `./${packageConfig.main}` : '';
@@ -769,6 +763,13 @@ function packageResolve(specifier, base, conditions) {
 
     // Package match.
     const packageConfig = packageJsonReader.read(packageJSONPath, { __proto__: null, specifier, base, isESM: true });
+    const invalid = packageConfig.main === undefined && packageConfig.exports === undefined;
+    if (invalid) {
+      throw new ERR_INVALID_PACKAGE_CONFIG(
+        fileURLToPath(packageJSONUrl), base,
+        'package.json must have a "main" or "exports" field'
+      );
+    }
     if (packageConfig.exports != null) {
       return packageExportsResolve(
         packageJSONUrl, packageSubpath, packageConfig, base, conditions);