Part 2 of Teaching Myself Some More React
Chapter 2: The React Rendering Lifecycle
Dhruv
Senior Frontend Developer specializing in React
Introduction: The Heart of React Rendering
At the core of any React application lies the rendering lifecycle — the sequence of processes that React uses to update the user interface (UI). For Server-Side Rendering (SSR), this lifecycle expands beyond the client, requiring developers to consider how rendering behaves both on the server and during hydration on the client.
This chapter dives into:
- How React renders components in CSR and SSR.
- Hydration: Why it’s critical and how it works.
- Lifecycle methods and hooks relevant to SSR.
- Key differences between server and client rendering lifecycles.
The React Rendering Process
1. Client-Side Rendering (CSR) Lifecycle
In CSR, rendering occurs entirely in the browser:
- Initial JavaScript Execution:
- React reads the component tree and virtual DOM structure.
- The virtual DOM is reconciled with the real DOM.
- State and Props Updates:
- React re-renders components when state or props change.
- Only the differences are patched into the DOM (diffing algorithm).
- UI Update:
- The browser reflects the updated UI for the user.
2. Server-Side Rendering (SSR) Lifecycle
In SSR, React skips the browser for the initial rendering:
- Rendering on the Server:
- Components are converted into HTML using
renderToString()
orrenderToPipeableStream()
. - React doesn’t use the DOM; instead, it generates a static string.
- Components are converted into HTML using
- Delivery to the Client:
- The server sends fully-rendered HTML to the browser.
- The browser renders the HTML for immediate display.
- Hydration on the Client:
- React binds event listeners and prepares the app for interactivity.
- The static HTML becomes a fully functional React app.
Hydration: Bridging SSR and CSR
What Is Hydration?
Hydration is the process where React:
- Scans the server-rendered HTML.
- Attaches React’s virtual DOM to the existing DOM.
- Makes the page interactive (e.g., event listeners).
Why Hydration Matters
Without hydration, SSR-only pages would display static content with no interactivity. Hydration ensures the seamless handoff from server-rendered HTML to a React-powered, dynamic application.
Hydration Process in Steps
- Browser receives the SSR HTML.
- React’s
ReactDOM.hydrateRoot()
method:- Matches the existing DOM with the virtual DOM.
- Updates only necessary components to avoid re-rendering the entire tree.
- Event listeners and other interactivity are attached.
Lifecycle Hooks in SSR
React’s lifecycle hooks behave differently during SSR. Let’s explore key lifecycle hooks and their roles:
1. constructor()
- Used for initializing state and binding methods.
- Executes only once on the server and again on the client during hydration.
2. componentDidMount()
- Not executed on the server.
- Runs after hydration on the client.
- Example Use: Fetching data that depends on client-specific APIs (e.g.,
window
orlocalStorage
).
3. componentWillUnmount()
- Relevant only on the client.
- Cleans up event listeners or other resources.
4. useEffect()
- Skipped during server rendering.
- Runs only on the client after hydration.
- SSR Pro Tip: Use
useEffect()
for browser-specific code like analytics or DOM manipulations.
5. getDerivedStateFromProps()
- Executes on both the server and the client.
- Ideal for deriving state based on props during SSR.
Differences Between Server and Client Rendering
Aspect | SSR | CSR |
---|---|---|
Initial Rendering | On the server, outputting HTML as a string. | In the browser, updating the DOM directly. |
Lifecycle Hooks | Limited hooks (e.g., no componentDidMount ). | All lifecycle hooks available. |
Data Fetching | Happens before rendering on the server. | Often done after rendering in the browser. |
Hydration | Required for interactivity. | Not applicable. |
Common Challenges with SSR Rendering Lifecycle
- Mismatched HTML:
- If server-rendered HTML doesn’t match the client-rendered output, React throws warnings.
- Fix: Ensure consistent data fetching and rendering logic on the server and client.
- Skipping Browser-Specific Code:
- Direct DOM manipulations or use of browser APIs (
window
,document
) during SSR will cause errors. - Fix: Wrap such logic in
useEffect()
or check fortypeof window
.
- Direct DOM manipulations or use of browser APIs (
- State Rehydration:
- Ensuring the server-rendered state matches the client’s initial state.
- Fix: Serialize and pass the state from the server to the client.
Pro Tips for Managing SSR Lifecycle
- Avoid Non-Serializable State:
- Ensure state objects are JSON-serializable for seamless transfer between server and client.
- Use Static Site Generation (SSG) for Rarely Updated Pages:
- Save SSR for dynamic content.
- Optimize Hydration Time:
- Minimize heavy computations during hydration by deferring them with
useEffect()
.
- Minimize heavy computations during hydration by deferring them with
- Leverage Streaming for Large Pages:
- Use React’s
renderToPipeableStream()
to progressively send HTML to the client.
- Use React’s
Key Takeaways
- React’s lifecycle hooks behave differently in SSR; plan your logic to account for server and client differences.
- Hydration bridges the gap between static server-rendered HTML and dynamic client-side React apps.
- Errors in SSR often stem from mismatched HTML or improper state serialization.
Quiz
- What is the role of hydration in SSR?
- Which lifecycle hooks are skipped during SSR, and why?
- What common mistakes lead to mismatched HTML warnings in SSR?
- Why does
useEffect
not run during SSR? - Explain how to handle browser-specific code in an SSR React app.
Practical Assignment
- Assignment: Implement a React app with SSR, rendering a list of blog posts on the server and hydrating it on the client. Add a button that filters posts dynamically after hydration.
- Expected Outcome: A fully server-rendered blog page that becomes interactive after hydration.
Part 2 of Teaching Myself Some More React
Dhruv
Dynamic Frontend Developer specializing in React.js and Next.js. Creating engaging web experiences with modern technologies and beautiful animations.