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Build a Minimal React Server-Side Rendering (SSR) Framework

This challenge asks you to build a simplified Server-Side Rendering (SSR) framework for React applications using TypeScript. This is crucial for improving initial load times, SEO, and user experience by rendering React components on the server before sending them to the client.

Problem Description

Your task is to create a basic SSR framework that can take a React component tree and render it to an HTML string on the server. The framework should also handle the initial hydration of the client-side React application, allowing it to take over the server-rendered HTML.

Key Requirements:

  1. Server-Side Rendering: Implement a function that accepts a React element (component tree) and returns its HTML representation.
  2. Client-Side Hydration: Implement a function that takes the server-rendered HTML element and "hydrates" it with a client-side React application, making it interactive.
  3. Basic Routing (Optional but Recommended): While not strictly required for the core SSR/hydration, consider how a very simple routing mechanism could be integrated to render different components based on a URL.
  4. TypeScript Support: The entire solution must be written in TypeScript, leveraging its type safety features.
  5. Minimal Dependencies: Aim for minimal external dependencies beyond React itself.

Expected Behavior:

  • Server:
    • Receive a React element representing the application's root.
    • Render this element into a complete HTML string.
    • Optionally, include a <script> tag to load client-side JavaScript that will perform hydration.
  • Client:
    • When the application loads, find the server-rendered HTML.
    • "Attach" the client-side React application to this existing HTML structure, preserving the DOM where possible and making it fully interactive.

Edge Cases to Consider:

  • Handling of different component types (functional, class-based).
  • Initial state management for client-side hydration.
  • Error handling during server rendering.

Examples

Example 1: Simple Page Rendering

Let's say you have a simple React component:

function Greeting({ name }) {
  return <h1>Hello, {name}!</h1>;
}
  • Input (Server-side): A React element, e.g., React.createElement(Greeting, { name: "World" }) and a DOCTYPE.
  • Output (Server-side HTML): 
    <!DOCTYPE html>
<html>
  <head>
    <title>My SSR App</title>
  </head>
  <body>
    <div id="root"><h1>Hello, World!</h1></div>
    <script src="/client.js"></script>
  </body>
</html>
  • Explanation: The Greeting component is rendered on the server into <h1>Hello, World!</h1> and placed within a div with id="root". A DOCTYPE and basic html, head, and body structure are prepended, along with a script tag for client-side code.

Example 2: Client Hydration

Assuming the HTML from Example 1 is received by the browser.

  • Input (Client-side): The div with id="root" containing <h1>Hello, World!</h1>.
  • Output (Client-side React state): The client-side React application is mounted onto the div with id="root", making the Greeting component interactive. No visible DOM changes occur immediately, but the component is now managed by React on the client.
  • Explanation: The client-side JavaScript runs, finds the div with id="root", and uses ReactDOM.hydrateRoot (or similar) to attach the Greeting component to the existing DOM, ensuring it's managed by React.

Constraints

  • React Version: Use React 18 or later for createRoot and hydrateRoot.
  • TypeScript Version: Use TypeScript 4.0 or later.
  • Dependencies: Allowed dependencies are react, react-dom, and potentially a lightweight routing library if you choose to implement it (e.g., react-router-dom but keep it minimal). Avoid full-fledged meta-frameworks like Next.js or Remix.
  • Server Environment: Assume a Node.js environment for server-side rendering.
  • Bundle Size: While not strictly enforced by automated tests, strive for an efficient and lean implementation.

Notes

  • You'll need to simulate a server environment. For development, you might use a simple Express.js server.
  • Consider how to pass data from the server to the client for hydration (e.g., embedding initial state in a <script> tag).
  • The goal is to understand the core mechanisms of SSR and hydration, not to build a production-ready framework with all the bells and whistles.
  • Think about the entry points for both server and client. How does the server know which component to render? How does the client know what to hydrate?
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