---
title: "Write Once, Test Everywhere: Cross-Platform Testing for React Native"
description: "Cross-platform testing in React Native made simple: a lightweight architecture to run and extend tests on any JavaScript-powered platform."
url: https://www.callstack.com/events/write-once-test-everywhere-cross-platform-testing-for-react-native
updated: 2026-10-24T08:39:22.922Z
---

# Write Once, Test Everywhere: Cross-Platform Testing for React Native

Cross-platform testing in React Native made simple: a lightweight architecture to run and extend tests on any JavaScript-powered platform.

_Talk · 10/4/2025, 12:30PM - 1PM [CET] · Alicante · Recording: 30:27 · Category: Testing_

Organizer: React Alicante

Speakers: Łukasz Chludziński (Engineering Manager @ Callstack)

[Watch the recording](https://www.youtube.com/watch?v=qmPH4v1KbJk)

Testing React Native apps has always meant choosing between speed and realism. Jest is fast, familiar, and great for unit tests; but it runs in Node.js with mocks for native modules. End-to-end frameworks give you realism, but at the cost of setup, maintenance, and slow pipelines.

At React Alicante 2025, Łukasz Chludziński introduces React Native Harness, a new testing framework created by Szymon Chmal at Callstack. Harness runs Jest-style tests directly on real devices and simulators, with full access to native modules and no mocks by default.

You'll see:

- How Harness plugs into Metro to run tests inside the real runtime
- Testing native libraries like MMKV without custom runners or hacks
- Verifying spec compliance for APIs like fetch and WebSocket
- How developers can keep their familiar Jest DX while gaining native access
- Why this fills the "missing middle ground" between unit tests and E2E