React on Rails 16.0.0 Release Notes

Note: Version 15.0.0 has been retracted. Please upgrade directly from v14 to v16.

Major Features

🚀 React Server Components Support

Experience the future of React with full RSC integration in your Rails apps:

Seamlessly use React Server Components
Reduce client bundle sizes
Enable powerful new patterns for data fetching
⚡️ Requires React on Rails Pro - See the full tutorial

🚀 Major Performance Breakthrough: Early Hydration

React on Rails now starts hydration even before the full page is loaded! This revolutionary change delivers significant performance improvements across all pages:

Eliminates Race Conditions: No more waiting for full page load before hydration begins
Faster Time-to-Interactive: Components hydrate as soon as their server-rendered HTML reaches the client
Streaming HTML Optimization: Perfect for modern streaming responses - components hydrate in parallel with page streaming
Async Script Safety: Can use async scripts without fear of race conditions
No More Defer Needed: The previous need for defer to prevent race conditions has been eliminated

This optimization is particularly impactful for:

Streamed pages where content loads progressively
Large pages with many components
Slow network conditions where every millisecond counts
Modern web apps requiring fast interactivity

Performance improvement visualization:

Performance comparison showing early hydration improvement

The image above demonstrates the dramatic performance improvement:

Left (Before): Hydration didn't start until the full page load completed, causing a huge delay before hydration
Right (After): Hydration starts immediately as soon as components are available, without waiting for full page load
Result: Components now become interactive much faster, eliminating the previous race condition delays

Enhanced Script Loading Strategies

New configuration option generated_component_packs_loading_strategy replaces defer_generated_component_packs
Supports three loading strategies:
- :async - Loads scripts asynchronously (default for Shakapacker ≥ 8.2.0)
- :defer - Defers script execution until after page load (doesn't work well with Streamed HTML as it will wait for the full page load before hydrating the components)
- :sync - Loads scripts synchronously (default for Shakapacker < 8.2.0) (better to upgrade to Shakapacker 8.2.0 and use :async strategy)
Improves page performance by optimizing how component packs are loaded

Breaking Changes

Component Hydration Changes

The defer_generated_component_packs configuration has been deprecated. Use generated_component_packs_loading_strategy instead.
The generated_component_packs_loading_strategy defaults to :async for Shakapacker ≥ 8.2.0 and :sync for Shakapacker < 8.2.0.
The immediate_hydration configuration now defaults to false. Note: immediate_hydration is a React on Rails Pro (licensed) feature.
When generated_component_packs_loading_strategy: :async and immediate_hydration: true are configured together, they optimize component hydration. Components hydrate as soon as their code and server-rendered HTML are available, without waiting for the full page to load. This parallel processing significantly improves time-to-interactive by eliminating the traditional waterfall of waiting for page load before beginning hydration (It's critical for streamed HTML).
- The previous need for deferring scripts to prevent race conditions has been eliminated due to improved hydration handling. Making scripts not defer is critical to execute the hydration scripts early before the page is fully loaded.
- The immediate_hydration configuration (React on Rails Pro licensed feature) makes react-on-rails hydrate components immediately as soon as their server-rendered HTML reaches the client, without waiting for the full page load.
- To enable optimized hydration, you can set immediate_hydration: true in your config/initializers/react_on_rails.rb file (requires React on Rails Pro license).
  - You can also enable it for individual components by passing immediate_hydration: true to react_component or stream_react_component.
- Redux store now supports the immediate_hydration option (React on Rails Pro licensed feature), which defaults to config.immediate_hydration (and so to false if that isn't set). If true, the Redux store will hydrate immediately as soon as its server-side data reaches the client.
  - You can override this behavior for individual Redux stores by calling the redux_store helper with immediate_hydration: true or immediate_hydration: false, same as react_component.
ReactOnRails.reactOnRailsPageLoaded() is now an async function:
- If you manually call this function to ensure components are hydrated (e.g., with async script loading), you must now await the promise it returns:
```
// Before
ReactOnRails.reactOnRailsPageLoaded();
// Code expecting all components to be hydrated

// After
await ReactOnRails.reactOnRailsPageLoaded();
// Code expecting all components to be hydrated
```
- If you call it in a turbolinks:load listener to work around the issue documented in Turbolinks, the listener can be safely removed.

Script Loading Strategy Migration

If you were previously using defer_generated_component_packs: true, use generated_component_packs_loading_strategy: :defer instead
If you were previously using defer_generated_component_packs: false, use generated_component_packs_loading_strategy: :sync instead
For optimal performance with Shakapacker ≥ 8.2.0, consider using generated_component_packs_loading_strategy: :async

ESM-only package

The package is now published as ES Modules instead of CommonJS. In most cases it shouldn't affect your code, as bundlers will be able to handle it. However:

If you explicitly use require('react-on-rails'), and can't change to import, upgrade to Node v20.19.0+ or v22.12.0+. They allow require for ESM modules without any flags. Node v20.17.0+ with --experimental-require-module should work as well.
If you run into TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. TypeScript error, you'll need to upgrade to TypeScript 5.8 and set module to nodenext.

Finally, if everything else fails, please contact us and we'll help you upgrade or release a dual ESM-CJS version.

`globalThis`

globalThis is now used in code. It should be available in browsers since 2020 and in Node, but in case your environment doesn't support it, you'll need to shim it using globalthis or core-js.

Store Dependencies for Components

When using Redux stores with multiple components, you need to explicitly declare store dependencies to optimize hydration. Here's how:

The Problem

If you have deferred Redux stores and components like this:

<% redux_store("SimpleStore", props: @app_props_server_render, defer: true) %>
<%= react_component('ReduxApp', {}, {prerender: true}) %>
<%= react_component('ComponentWithNoStore', {}, {prerender: true}) %>
<%= redux_store_hydration_data %>

By default, React on Rails assumes components depend on all previously created stores. This means:

Neither ReduxApp nor ComponentWithNoStore will hydrate until SimpleStore is hydrated
Since the store is deferred to the end of the page, both components are forced to wait unnecessarily

The Solution

Explicitly declare store dependencies for each component:

<% redux_store("SimpleStore", props: @app_props_server_render, defer: true) %>
<%= react_component('ReduxApp', {}, {
  prerender: true
  # No need to specify store_dependencies: it automatically depends on SimpleStore
}) %>
<%= react_component('ComponentWithNoStore', {}, {
  prerender: true,
  # Explicitly declare no store dependencies
  store_dependencies: []
}) %>
<%= redux_store_hydration_data %>

This allows ComponentWithNoStore to hydrate immediately without waiting for SimpleStore, improving page performance.