From b5ee33c823289ce3d39d7e060f5c886f7a2fffaf Mon Sep 17 00:00:00 2001
From: Siddhartha Singh <siddharthagithub0007@gmail.com>
Date: Sat, 7 Mar 2026 08:20:39 +0530
Subject: [PATCH] test_runner: sync ESM exports when mocking timers

This change ensures that ESM imports of node:timers/promises are correctly
mocked when using mock.timers.enable(). This is achieved by calling
syncBuiltinESMExports() whenever the mocked timers are toggled or reset.

Fixes: https://github.com/nodejs/node/issues/62081
---
 lib/internal/test_runner/mock/mock_timers.js  |  9 ++--
 test/parallel/test-runner-mock-timers-esm.mjs | 44 +++++++++++++++++++
 2 files changed, 50 insertions(+), 3 deletions(-)
 create mode 100644 test/parallel/test-runner-mock-timers-esm.mjs

diff --git a/lib/internal/test_runner/mock/mock_timers.js b/lib/internal/test_runner/mock/mock_timers.js
index 643128c3f0031f..666998c023490b 100644
--- a/lib/internal/test_runner/mock/mock_timers.js
+++ b/lib/internal/test_runner/mock/mock_timers.js
@@ -24,6 +24,8 @@ const {
   validateNumber,
   validateStringArray,
 } = require('internal/validators');
+const { Module } = require('internal/modules/cjs/loader');
+const { syncBuiltinESMExports } = Module;
 
 const {
   AbortError,
@@ -440,9 +442,9 @@ class MockTimers {
 
     const eventIt = EventEmitter.on(emitter, 'data');
     const timer = this.#createTimer(true,
-                                    () => emitter.emit('data'),
-                                    interval,
-                                    options);
+      () => emitter.emit('data'),
+      interval,
+      options);
 
     try {
       // eslint-disable-next-line no-unused-vars
@@ -628,6 +630,7 @@ class MockTimers {
     const target = activate ? options.toFake : options.toReal;
     ArrayPrototypeForEach(this.#timersInContext, (timer) => target[timer]());
     this.#isEnabled = activate;
+    syncBuiltinESMExports();
   }
 
   /**
diff --git a/test/parallel/test-runner-mock-timers-esm.mjs b/test/parallel/test-runner-mock-timers-esm.mjs
new file mode 100644
index 00000000000000..2eb1955ead5da7
--- /dev/null
+++ b/test/parallel/test-runner-mock-timers-esm.mjs
@@ -0,0 +1,44 @@
+import { describe, it } from 'node:test';
+import { setTimeout } from 'node:timers/promises';
+import assert from 'node:assert';
+
+describe('Mock Timers ESM Regression', () => {
+    it('should work with node:timers/promises and runAll() in ESM', async (t) => {
+        t.mock.timers.enable({ apis: ['Date', 'setTimeout'] });
+        const startTime = Date.now();
+        let t1 = 0;
+
+        await Promise.all([
+            (async () => {
+                await setTimeout(1000);
+                t1 = Date.now();
+            })(),
+            (async () => {
+                // Wait for the next tick to ensure setTimeout has been called
+                await new Promise(resolve => process.nextTick(resolve));
+                t.mock.timers.runAll();
+            })()
+        ]);
+
+        assert.strictEqual(t1 - startTime, 1000);
+    });
+
+    it('should work with node:timers/promises and tick() in ESM', async (t) => {
+        t.mock.timers.enable({ apis: ['Date', 'setTimeout'] });
+        const startTime = Date.now();
+        let t1 = 0;
+
+        await Promise.all([
+            (async () => {
+                await setTimeout(500);
+                t1 = Date.now();
+            })(),
+            (async () => {
+                await new Promise(resolve => process.nextTick(resolve));
+                t.mock.timers.tick(500);
+            })()
+        ]);
+
+        assert.strictEqual(t1 - startTime, 500);
+    });
+});