Testing
30:27
10/4/2025

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

Presented
React Alicante
Speaker
Łukasz Chludziński
Engineering Manager
@
Callstack

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
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