Aside from the original API Client, we have a React-specific package which eliminates the need for extension developers to do a lot of boilerplate setup when using React for Extension development. The package comes with a set of hooks and components that combine to make a blazingly fast and seamless developer experience.

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Installation

npm install @replit/extensions-react
yarn add @replit/extensions-react
pnpm add @replit/extensions-react

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Usage

Fork the React Extension Template to get started. Alternatively, you can start from scratch by wrapping your application with the HandshakeProvider component imported from @replit/extensions-react. 
import { HandshakeProvider } from '@replit/extensions-react';
import { createRoot } from 'react-dom/client';
import App from './App';

createRoot(document.getElementById('root')).render(
  <HandshakeProvider>
    <App />
  </HandshakeProvider>
)
In the App function, check the handshake status with the useReplit hook. 
import { useReplit } from '@replit/extensions-react';

function App() {
  const { status, error, replit } = useReplit();

  if(status === "loading") {
    return <div>Loading...</div>
  }

  if(status === "error") {
    return <div>An error occurred: {error?.message}</div>
  }

  return <div>
    Extension is Ready!
  </div>
}