No New Features, Just Improvements

React 17 focused on improving the existing architecture rather than introducing new features.

Improved Event Delegation

Events are now attached to the root rather than document.

// Before React 17
// Event listeners were attached to document
ReactDOM.render(<App />, rootNode);

// In React 17
// Event listeners are attached to the root DOM container
ReactDOM.render(<App />, rootNode);

Gradual Upgrades

Better compatibility with older versions of React.

// React 17 app
const root = document.getElementById('root');
ReactDOM.render(<App />, root);

// React 18 app inside React 17 app
const newRoot = document.getElementById('new-root');
// Can use React 18 features here
ReactDOM.createRoot(newRoot).render(<NewApp />);

JSX Transform Update

Allows using JSX without explicitly importing React.

// Before React 17
import React from 'react';

function App() {
  return <h1>Hello World</h1>;
}

// After React 17
// No need to import React when using JSX
function App() {
  return <h1>Hello World</h1>;
}