Part 2 · Fundamentals

What JSI changed

Prerequisites: 01-three-layers

What JSI changed — mechanism

Two conceptual interop paths; the lower path removes mandatory bridge serialization, not every copy or scheduling cost. Nodes: Legacy JS, Serialized queue, Native, New JS, C++ host object, Native.
Two conceptual interop paths; the lower path removes mandatory bridge serialization, not every copy or scheduling cost. [S01] [S07]

Mental model verified

The legacy bridge queued serialized messages between JS and native. In the New Architecture, JSI lets JS interact with C++ host objects through an engine-independent interface. Native module frameworks build typed or declarative APIs above it.

[S01] [S07]

Worked example synthesis

A synchronous getCachedValue() may return a small cached scalar. A synchronous readHugeFile() would block the JS caller until I/O finishes; put I/O in an asynchronous function. This is an illustrative design example.

[S07]

Engineering choice synthesis

Keep synchronous APIs bounded and predictable. Pass identifiers or handles for large native assets where a library supports them; measure conversion and end-to-end latency rather than guessing from call count.

[S01] [S07]

Pitfall and next step synthesis

“Zero-copy everywhere” is false: JSI permits native references but APIs can still convert or copy values. Profile the real path and check ownership and lifetime of referenced native data.

[S01] [S07]
Keep this: JSI allows direct runtime bindings and native object references, but synchronous calls still consume caller time.
Check yourself: Does “no old bridge” mean there is no boundary cost?

No. JSI avoids mandatory JSON-style message serialization, yet conversion, allocation, locks, scheduling and native work can still cost time.

Sources & further reading

  1. [S01] About the New Architecture

    React Native · official documentation · accessed 2026-10-08 · current documentation as accessed

    Supports: JSI replaces the serialized bridge; enabling it alone does not promise speed.

    Read the diagrams and API boundaries for jsi replaces the serialized bridge; enabling it alone does not promise speed.

  2. [S07] Expo Modules API: Reference

    Expo · official documentation · accessed 2026-10-08 · current documentation as accessed

    Supports: JSI abstraction; Function, AsyncFunction, queues, views, events and lifecycle.

    Read the diagrams and API boundaries for jsi abstraction; function, asyncfunction, queues, views, events and lifecycle.