esm: do not call getSource when format is commonjs

Ensure that `defaultLoad` does not uselessly access the file system to get the source of modules that are known to be in CommonJS format. This allows CommonJS imports to resolve in the current phase of the event loop. Refs: eslint/eslint#17683 PR-URL: #50465 Reviewed-By: James M Snell <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]>

Author: fasttime

lib/internal/modules/esm/load.js

@@ -132,20 +132,21 @@ async function defaultLoad(url, context = kEmptyObject){
 if(urlInstance.protocol==='node:'){
 source=null;
 format??='builtin';
-}else{
-letcontextToPass=context;
+}elseif(format!=='commonjs'||defaultType==='module'){
 if(source==null){
 ({ responseURL, source }=awaitgetSource(urlInstance,context));
-contextToPass={__proto__: context, source };
+context={__proto__: context, source };
 }
 
-// Now that we have the source for the module, run `defaultGetFormat` again in case we detect ESM syntax.
-format??=awaitdefaultGetFormat(urlInstance,contextToPass);
+if(format==null){
+// Now that we have the source for the module, run `defaultGetFormat` to detect its format.
+format=awaitdefaultGetFormat(urlInstance,context);
 
-if(format==='commonjs'&&contextToPass!==context&&defaultType!=='module'){
-// For backward compatibility reasons, we need to discard the source in
-// order for the CJS loader to re-fetch it.
-source=null;
+if(format==='commonjs'){
+// For backward compatibility reasons, we need to discard the source in
+// order for the CJS loader to re-fetch it.
+source=null;
+}
 }
 }
 

test/es-module/test-esm-dynamic-import-commonjs.js

@@ -0,0 +1,19 @@
+'use strict';
+
+constcommon=require('../common');
+constfixtures=require('../common/fixtures');
+constassert=require('node:assert');
+
+(async()=>{
+
+// Make sure that the CommonJS module lexer has been initialized.
+// See https://github.com/nodejs/node/blob/v21.1.0/lib/internal/modules/esm/translators.js#L61-L81.
+awaitimport(fixtures.fileURL('empty.js'));
+
+lettickDuringCJSImport=false;
+process.nextTick(()=>{tickDuringCJSImport=true;});
+awaitimport(fixtures.fileURL('empty.cjs'));
+
+assert(!tickDuringCJSImport);
+
+})().then(common.mustCall());

test/es-module/test-esm-dynamic-import-commonjs.mjs

@@ -0,0 +1,9 @@
+import'../common/index.mjs';
+importfixturesfrom'../common/fixtures.js';
+importassertfrom'node:assert';
+
+lettickDuringCJSImport=false;
+process.nextTick(()=>{tickDuringCJSImport=true;});
+awaitimport(fixtures.fileURL('empty.cjs'));
+
+assert(!tickDuringCJSImport);

test/es-module/test-esm-loader-with-source.mjs

@@ -0,0 +1,10 @@
+// Flags: --experimental-loader ./test/fixtures/es-module-loaders/preset-cjs-source.mjs
+import'../common/index.mjs';
+import*asfixturesfrom'../common/fixtures.mjs';
+importassertfrom'assert';
+
+const{default: existingFileSource}=awaitimport(fixtures.fileURL('es-modules','cjs-file.cjs'));
+const{default: noSuchFileSource}=awaitimport(newURL('./no-such-file.cjs',import.meta.url));
+
+assert.strictEqual(existingFileSource,'no .cjs file was read to get this source');
+assert.strictEqual(noSuchFileSource,'no .cjs file was read to get this source');

test/fixtures/empty.cjs

@@ -0,0 +1,21 @@
+exportfunctionresolve(specifier,context,next){
+if(specifier.endsWith('/no-such-file.cjs')){
+// Shortcut to avoid ERR_MODULE_NOT_FOUND for non-existing file, but keep the url for the load hook.
+return{
+shortCircuit: true,
+url: specifier,
+};
+}
+returnnext(specifier);
+}
+
+exportfunctionload(href,context,next){
+if(href.endsWith('.cjs')){
+return{
+format: 'commonjs',
+shortCircuit: true,
+source: 'module.exports = "no .cjs file was read to get this source"',
+};
+}
+returnnext(href);
+}