What’s New in React 19?

Introduction

React 19 emerges as a game-changer in front-end development, introducing many innovative features. From refined concurrent rendering to novel state-handling mechanisms, React 19 enhances both performance and developer experience. In this article, we explore these cutting-edge additions, providing insights into how they reshape the landscape of dynamic user interface development. Whether you're a seasoned React developer or new to the framework, understanding React 19's advancements is crucial for staying ahead in modern web development. Join us on a journey to unravel the transformative features that make React 19 a milestone release, setting the stage for the future of front-end innovation.

New features

React Compiler

React Compiler, no longer a research project, is now powering Instagram.com in production, representing a significant advancement in React's capabilities. It addresses the issue of excessive re-renders on state changes by introducing an optimizing compiler. Unlike manual memoization, this compiler automatically re-renders specific UI parts when the state changes, eliminating code clutter. Operating within the rules of both JavaScript and React, it ensures safety and performance. Developers can validate their code with tools like Strict Mode and React's ESLint plugin. The compiler is already active on Instagram.com and is set for further integration across Meta surfaces, with plans for an open-source release.

Actions

Actions allow you to pass a function to DOM elements such as

<form action={search}>
  <input name="query" />
  <button type="submit">Search</button>
</form>

The action function has the flexibility to operate synchronously or asynchronously. It can be defined on the client side using standard JavaScript or on the server using the 'use server' directive. React takes charge of managing the data submission life cycle when an action is employed, offering hooks like useFormStatus and useFormState for accessing the current form action's state and response.

import { useFormState } from "react-dom";

async function increment(previousState, formData) {
  return previousState + 1;
}

function StatefulForm({}) {
  const [state, formAction] = useFormState(increment, 0);
  return (
    <form>
      {state}
      <button formAction={formAction}>Increment</button>
    </form>
  )
}

By default, Actions are submitted within a transition, maintaining the interactivity of the current page during processing. With support for async functions, the introduction of async/await in transitions enables the display of pending UI, utilizing the isPending state to signal ongoing processing during asynchronous requests like fetching data.

React Canary

Canaries mark a shift in our React development approach. Unlike the previous method where features were researched and developed privately within Meta, Canaries involved building in public with community collaboration to refine features showcased in the React Labs blog series. This approach ensures that users are informed about upcoming features earlier in the development process. It allows them to witness the finalization stages rather than only encountering the polished product upon Stable release.

Some of the features in the react canary are
React Server Components, Asset Loading, Document Metadata, and Actions

Directives

"use client" and "use server" are bundler features designed for full-stack React frameworks. They mark the “split points” between the two environments: "use client" instructs the bundler to generate a <script> tag (like Astro Islands), while "use server" tells the bundler to generate a POST endpoint (like tRPC Mutations). Together, they let you write reusable components that compose client-side interactivity with the related server-side logic.

Document Metadata:

Support for rendering <title>, <meta>, and metadata <link> tags anywhere in your component tree has been added. These work the same way in all environments, including fully client-side code, SSR, and RSC. This provides built-in support for features pioneered by libraries like React Helmet.

Actions

Actions are to manage sending data from the client to the server. You can add action to elements like <form/>, access the status with useFormStatus, handle the result with useFormState, and optimistically update the UI with useOptimistic.

import { useOptimistic } from 'react';

function AppContainer() {
  const [optimisticState, addOptimistic] = useOptimistic(
    state,
    // updateFn
    (currentState, optimisticValue) => {
      // merge and return new state
      // with optimistic value
    }
  );
}

To get started with the React canary

//for npm 
npm install react@canary react-dom@canary
//for yarn
yarn add react@canary react-dom@canary