test_runner: mock_loader resolve the cjs and esm formats respectively.

lib/test/mock_loader.js

@@ -22,7 +22,9 @@ const { normalizeReferrerURL } = require('internal/modules/helpers');
 let debug = require('internal/util/debuglog').debuglog('test_runner', (fn) => {
   debug = fn;
 });
-const { createRequire, isBuiltin } = require('module');
+const { createRequire, isBuiltin, _resolveFilename } = require('module');
+const { defaultGetFormatWithoutErrors } = require('internal/modules/esm/get_format');
+const { defaultResolve } = require('internal/modules/esm/resolve');
 
 // TODO(cjihrig): This file should not be exposed publicly, but register() does
 // not handle internal loaders. Before marking this API as stable, one of the
@@ -95,11 +97,20 @@ async function resolve(specifier, context, nextResolve) {
   if (isBuiltin(specifier)) {
     mockSpecifier = ensureNodeScheme(specifier);
   } else {
-    // TODO(cjihrig): This try...catch should be replaced by defaultResolve(),
-    // but there are some edge cases that caused the tests to fail on Windows.
+    let format;
+
+    if(context.parentURL) {
+      format = defaultGetFormatWithoutErrors(pathToFileURL(context.parentURL));
+    }
+
     try {
-      const req = createRequire(context.parentURL);
-      specifier = pathToFileURL(req.resolve(specifier)).href;
+      if (format === 'module') {
+        specifier = defaultResolve(specifier, context).url;
+      } else {
+        specifier = pathToFileURL(
+          createRequire(context.parentURL).resolve(specifier)
+        ).href;
+      }
     } catch {
       const parentURL = normalizeReferrerURL(context.parentURL);
       const parsedURL = URL.parse(specifier, parentURL)?.href;

test/fixtures/module-mocking/basic-esm-without-extension.js

@@ -0,0 +1 @@
+export let string = 'original esm string';

test/fixtures/module-mocking/wrong-import-after-module-mocking.js

@@ -0,0 +1,8 @@
+import { mock } from 'node:test';
+
+try {
+  if (mock.module) mock.module('Whatever, this is not significant', { namedExports: {} });
+} catch {}
+
+const { string } = await import('./basic-esm-without-extension');
+console.log(`Found string: ${string}`); // prints 'original esm string'

test/parallel/test-runner-mocking.js

@@ -2,6 +2,7 @@
 const common = require('../common');
 const assert = require('node:assert');
 const { mock, test } = require('node:test');
+const fixtures = require('../common/fixtures');
 test('spies on a function', (t) => {
   const sum = t.mock.fn((arg1, arg2) => {
     return arg1 + arg2;
@@ -1054,3 +1055,17 @@ test('setter() fails if getter options is true', (t) => {
     t.mock.setter({}, 'method', { getter: true });
   }, /The property 'options\.setter' cannot be used with 'options\.getter'/);
 });
+
+test('wrong import syntax should throw error after module mocking.', async () => {
+  const { stdout, stderr } = await common.spawnPromisified(
+    process.execPath,
+    [
+      '--experimental-test-module-mocks',
+      '--experimental-default-type=module',
+      fixtures.path('module-mocking/wrong-import-after-module-mocking.js'),
+    ]
+  );
+
+  assert.equal(stdout.toString(), '')
+  assert.match(stderr.toString(), /Error \[ERR_MODULE_NOT_FOUND\]: Cannot find module/)
+});