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 <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

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 {
-    let contextToPass = context;
+  } else if (format !== 'commonjs' || defaultType === 'module') {
     if (source == null) {
       ({ responseURL, source } = await getSource(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 ??= await defaultGetFormat(urlInstance, contextToPass);
+    if (format == null) {
+      // Now that we have the source for the module, run `defaultGetFormat` to detect its format.
+      format = await defaultGetFormat(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';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const assert = 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.
+  await import(fixtures.fileURL('empty.js'));
+
+  let tickDuringCJSImport = false;
+  process.nextTick(() => { tickDuringCJSImport = true; });
+  await import(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';
+import fixtures from '../common/fixtures.js';
+import assert from 'node:assert';
+
+let tickDuringCJSImport = false;
+process.nextTick(() => { tickDuringCJSImport = true; });
+await import(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 * as fixtures from '../common/fixtures.mjs';
+import assert from 'assert';
+
+const { default: existingFileSource } = await import(fixtures.fileURL('es-modules', 'cjs-file.cjs'));
+const { default: noSuchFileSource } = await import(new URL('./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/es-module-loaders/preset-cjs-source.mjs

@@ -0,0 +1,21 @@
+export function resolve(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,
+    };
+  }
+  return next(specifier);
+}
+
+export function load(href, context, next) {
+  if (href.endsWith('.cjs')) {
+    return {
+      format: 'commonjs',
+      shortCircuit: true,
+      source: 'module.exports = "no .cjs file was read to get this source";',
+    };
+  }
+  return next(href);
+}