esm: do not give wrong hints when detecting file format

PR-URL: #50314
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

Author: aduh95

lib/internal/modules/esm/get_format.js

@@ -110,8 +110,10 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
 default: {// The user did not pass `--experimental-default-type`.
 // `source` is undefined when this is called from `defaultResolve`;
 // but this gets called again from `defaultLoad`/`defaultLoadSync`.
-if(source&&getOptionValue('--experimental-detect-module')){
-returncontainsModuleSyntax(`${source}`,fileURLToPath(url)) ? 'module' : 'commonjs';
+if(getOptionValue('--experimental-detect-module')){
+returnsource ?
+(containsModuleSyntax(`${source}`,fileURLToPath(url)) ? 'module' : 'commonjs') :
+null;
 }
 return'commonjs';
 }
@@ -136,9 +138,13 @@ function getFileProtocolModuleFormat(url, context = { __proto__: null }, ignoreE
 return'commonjs';
 }
 default: {// The user did not pass `--experimental-default-type`.
-if(source&&getOptionValue('--experimental-detect-module')&&
-getFormatOfExtensionlessFile(url)==='module'){
-returncontainsModuleSyntax(`${source}`,fileURLToPath(url)) ? 'module' : 'commonjs';
+if(getOptionValue('--experimental-detect-module')){
+if(!source){returnnull;}
+constformat=getFormatOfExtensionlessFile(url);
+if(format==='module'){
+returncontainsModuleSyntax(`${source}`,fileURLToPath(url)) ? 'module' : 'commonjs';
+}
+returnformat;
 }
 return'commonjs';
 }

lib/internal/modules/esm/load.js

@@ -105,7 +105,7 @@ function getSourceSync(url, context) {
  * @param {LoadContext} context
  * @returns {LoadReturn}
  */
-asyncfunctiondefaultLoad(url,context={__proto__: null}){
+asyncfunctiondefaultLoad(url,context=kEmptyObject){
 letresponseURL=url;
 let{
  importAttributes,
@@ -129,18 +129,22 @@ async function defaultLoad(url, context = { __proto__: null }) {
 
 if(urlInstance.protocol==='node:'){
 source=null;
-}elseif(source==null){
-({ responseURL, source }=awaitgetSource(urlInstance,context));
-context.source=source;
-}
+format??='builtin';
+}else{
+letcontextToPass=context;
+if(source==null){
+({ responseURL, source }=awaitgetSource(urlInstance,context));
+contextToPass={__proto__: context, source };
+}
 
-if(format==null||format==='commonjs'){
 // Now that we have the source for the module, run `defaultGetFormat` again in case we detect ESM syntax.
-format=awaitdefaultGetFormat(urlInstance,context);
-}
+format??=awaitdefaultGetFormat(urlInstance,contextToPass);
 
-if(format==='commonjs'){
-source=null;// Let the CommonJS loader handle it (for now)
+if(format==='commonjs'&&contextToPass!==context){
+// For backward compatibility reasons, we need to discard the source in
+// order for the CJS loader to re-fetch it.
+source=null;
+}
 }
 
 validateAttributes(url,format,importAttributes);

test/es-module/test-esm-detect-ambiguous.mjs

@@ -152,6 +152,29 @@ describe('--experimental-detect-module', { concurrency: true }, () => {
 strictEqual(signal,null);
 });
 }
+
+it('should not hint wrong format in resolve hook',async()=>{
+letwriteSync;
+const{ stdout, stderr, code, signal }=awaitspawnPromisified(process.execPath,[
+'--experimental-detect-module',
+'--no-warnings',
+'--loader',
+`data:text/javascript,import { writeSync } from "node:fs"; export ${encodeURIComponent(
+asyncfunctionresolve(s,c,next){
+constresult=awaitnext(s,c);
+writeSync(1,result.format+'\n');
+returnresult;
+}
+)}`,
+fixtures.path('es-modules/package-without-type/noext-esm'),
+]);
+
+strictEqual(stderr,'');
+strictEqual(stdout,'null\nexecuted\n');
+strictEqual(code,0);
+strictEqual(signal,null);
+
+});
 });
 
 describe('file input in a "type": "commonjs" package',{concurrency: true},()=>{