React's experimental_SuspenseList: Mastering Suspense Loading Patterns

React 16.6 introduced Suspense, a powerful mechanism for handling asynchronous data fetching in components. It provides a declarative way to display loading states while waiting for data. Building upon this foundation, experimental_SuspenseList offers even more control over the order in which content is revealed, particularly useful when dealing with lists or grids of data that load asynchronously. This blog post delves deep into experimental_SuspenseList, exploring its loading strategies and how to leverage them to create a superior user experience. While still experimental, understanding its principles will give you a head start when it graduates to a stable API.

Understanding Suspense and its Role

Before diving into experimental_SuspenseList, let's recap Suspense. Suspense allows a component to "suspend" rendering while waiting for a promise to resolve, typically a promise returned from a data fetching library. You wrap the suspending component with a <Suspense> component, providing a fallback prop that renders a loading indicator. This simplifies handling loading states and makes your code more declarative.

Basic Suspense Example:

Consider a component that fetches user data:

// Data Fetching (Simplified)
const fetchData = (userId) => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve({ id: userId, name: `User ${userId}`, country: 'Exampleland' });
    }, 1000);
  });
};

const UserProfile = ({ userId }) => {
  const userData = use(fetchData(userId)); // use() is part of React Concurrent Mode
  return (
    <div>
      <h2>{userData.name}</h2>
      <p>Country: {userData.country}</p>
    </div>
  );
};

const App = () => {
  return (
    <Suspense fallback={<p>Loading user profile...</p>}>
      <UserProfile userId={123} />
    </Suspense>
  );
};

In this example, UserProfile suspends while fetchData resolves. The <Suspense> component displays "Loading user profile..." until the data is ready.

Introducing experimental_SuspenseList: Orchestrating Loading Sequences

experimental_SuspenseList takes Suspense a step further. It allows you to control the order in which multiple Suspense boundaries are revealed. This is extremely useful when rendering lists or grids of items that load independently. Without experimental_SuspenseList, the items might appear in a jumbled order as they load, which can be visually jarring for the user. experimental_SuspenseList allows you to present content in a more coherent and predictable manner.

Key Benefits of using experimental_SuspenseList:

• Improved Perceived Performance: By controlling the reveal order, you can prioritize critical content or ensure a visually pleasing loading sequence, making the application feel faster.
• Enhanced User Experience: A predictable loading pattern is less distracting and more intuitive for users. It reduces the cognitive load and makes the application feel more polished.
• Reduced Layout Shifts: By managing the order of content appearing, you can minimize unexpected layout shifts as elements load, improving the overall visual stability of the page.
• Prioritization of Important Content: Show important elements first to keep the user engaged and informed.

Loading Strategies with experimental_SuspenseList

experimental_SuspenseList provides props to define the loading strategy. The two primary props are revealOrder and tail.

1. revealOrder: Defining the Reveal Order

The revealOrder prop determines the order in which the Suspense boundaries within the experimental_SuspenseList are revealed. It accepts three values:

• forwards: Reveals the Suspense boundaries in the order they appear in the component tree (top to bottom, left to right).
• backwards: Reveals the Suspense boundaries in the reverse order they appear in the component tree.
• together: Reveals all the Suspense boundaries at the same time, once all of them have loaded.

Example: Forwards Reveal Order

This is the most common and intuitive strategy. Imagine displaying a list of articles. You'd want the articles to appear from top to bottom as they load.

import { unstable_SuspenseList as SuspenseList } from 'react';

const Article = ({ articleId }) => {
  const articleData = use(fetchArticleData(articleId));
  return (
    <div>
      <h3>{articleData.title}</h3>
      <p>{articleData.content.substring(0, 100)}...</p>
    </div>
  );
};

const ArticleList = ({ articleIds }) => {
  return (
    <SuspenseList revealOrder="forwards">
      {articleIds.map(id => (
        <Suspense key={id} fallback={<p>Loading article {id}...</p>}>
          <Article articleId={id} />
        </Suspense>
      ))}
    </SuspenseList>
  );
};

//Usage
const App = () => {
 return (
   <Suspense fallback={<p>Loading articles...</p>}>
     <ArticleList articleIds={[1, 2, 3, 4, 5]} />
   </Suspense>
 );
};

In this example, the articles will load and appear on the screen in the order of their articleId, from 1 to 5.

Example: Backwards Reveal Order

This is useful when you want to prioritize the last items in a list, perhaps because they contain more recent or relevant information. Imagine displaying a reverse chronological feed of updates.

import { unstable_SuspenseList as SuspenseList } from 'react';

const Update = ({ updateId }) => {
  const updateData = use(fetchUpdateData(updateId));
  return (
    <div>
      <h3>{updateData.title}</h3>
      <p>{updateData.content.substring(0, 100)}...</p>
    </div>
  );
};

const UpdateFeed = ({ updateIds }) => {
  return (
    <SuspenseList revealOrder="backwards">
      {updateIds.map(id => (
        <Suspense key={id} fallback={<p>Loading update {id}...</p>}>
          <Update updateId={id} />
        </Suspense>
      ))}
    </SuspenseList>
  );
};

//Usage
const App = () => {
 return (
   <Suspense fallback={<p>Loading updates...</p>}>
     <UpdateFeed updateIds={[1, 2, 3, 4, 5]} />
   </Suspense>
 );
};

In this example, the updates will load and appear on the screen in the reverse order of their updateId, from 5 to 1.

Example: Together Reveal Order

This strategy is suitable when you want to present a complete set of data at once, avoiding any incremental loading. This can be useful for dashboards or views where a complete picture is more important than immediate partial information. However, be mindful of the overall loading time, as the user will see a single loading indicator until all data is ready.

import { unstable_SuspenseList as SuspenseList } from 'react';

const DataPoint = ({ dataPointId }) => {
  const data = use(fetchDataPoint(dataPointId));
  return (
    <div>
      <p>Data Point {dataPointId}: {data.value}</p>
    </div>
  );
};

const Dashboard = ({ dataPointIds }) => {
  return (
    <SuspenseList revealOrder="together">
      {dataPointIds.map(id => (
        <Suspense key={id} fallback={<p>Loading data point {id}...</p>}>
          <DataPoint dataPointId={id} />
        </Suspense>
      ))}
    </SuspenseList>
  );
};

//Usage
const App = () => {
 return (
   <Suspense fallback={<p>Loading dashboard...</p>}>
     <Dashboard dataPointIds={[1, 2, 3, 4, 5]} />
   </Suspense>
 );
};

In this example, the entire dashboard will remain in a loading state until all data points (1 to 5) have been loaded. Then, all data points will appear simultaneously.

2. tail: Handling Remaining Items After Initial Load

The tail prop controls how the remaining items in a list are revealed after the initial set of items has loaded. It accepts two values:

• collapsed: Hides the remaining items until all preceding items have loaded. This creates a "waterfall" effect, where items appear one after another.
• suspended: Suspends the rendering of the remaining items, showing their respective fallbacks. This allows for parallel loading but respects the revealOrder.

If tail is not provided, it defaults to collapsed.

Example: Collapsed Tail

This is the default behavior and often a good choice for lists where the order is important. It ensures that items appear in the specified order, creating a smooth and predictable loading experience.

import { unstable_SuspenseList as SuspenseList } from 'react';

const Item = ({ itemId }) => {
  const itemData = use(fetchItemData(itemId));
  return (
    <div>
      <h3>Item {itemId}</h3>
      <p>Description of item {itemId}.</p>
    </div>
  );
};

const ItemList = ({ itemIds }) => {
  return (
    <SuspenseList revealOrder="forwards" tail="collapsed">
      {itemIds.map(id => (
        <Suspense key={id} fallback={<p>Loading item {id}...</p>}>
          <Item itemId={id} />
        </Suspense>
      ))}
    </SuspenseList>
  );
};

//Usage
const App = () => {
 return (
   <Suspense fallback={<p>Loading items...</p>}>
     <ItemList itemIds={[1, 2, 3, 4, 5]} />
   </Suspense>
 );
};

In this example, with revealOrder="forwards" and tail="collapsed", each item will load sequentially. Item 1 loads first, then item 2, and so on. The loading state will “cascade” down the list.

Example: Suspended Tail

This allows for parallel loading of items while still respecting the overall reveal order. It's useful when you want to load items quickly but maintain some visual consistency. However, it might be slightly more visually distracting than the collapsed tail because multiple loading indicators might be visible at once.

import { unstable_SuspenseList as SuspenseList } from 'react';

const Product = ({ productId }) => {
  const productData = use(fetchProductData(productId));
  return (
    <div>
      <h3>{productData.name}</h3>
      <p>Price: {productData.price}</p>
    </div>
  );
};

const ProductList = ({ productIds }) => {
  return (
    <SuspenseList revealOrder="forwards" tail="suspended">
      {productIds.map(id => (
        <Suspense key={id} fallback={<p>Loading product {id}...</p>}>
          <Product productId={id} />
        </Suspense>
      ))}
    </SuspenseList>
  );
};

//Usage
const App = () => {
 return (
   <Suspense fallback={<p>Loading products...</p>}>
     <ProductList productIds={[1, 2, 3, 4, 5]} />
   </Suspense>
 );
};

In this example, with revealOrder="forwards" and tail="suspended", all products will start loading in parallel. However, they will still appear on the screen in order (1 to 5). You'll see the loading indicators for all items, and then they'll resolve in the correct sequence.

Practical Examples and Use Cases

Here are some real-world scenarios where experimental_SuspenseList can significantly improve the user experience:

• E-commerce Product Listings: Display products in a consistent order (e.g., based on popularity or relevance) as they load. Use revealOrder="forwards" and tail="collapsed" for a smooth, sequential reveal.
• Social Media Feeds: Show the most recent updates first using revealOrder="backwards". The tail="collapsed" strategy can prevent the page from jumping around as new posts load.
• Image Galleries: Present images in a visually appealing order, perhaps revealing them in a grid pattern. Experiment with different revealOrder values to achieve the desired effect.
• Data Dashboards: Load critical data points first to provide users with an overview, even if other sections are still loading. Consider using revealOrder="together" for components that need to be fully loaded before being displayed.
• Search Results: Prioritize the most relevant search results by ensuring they load first using revealOrder="forwards" and carefully ordered data.
• Internationalized Content: If you have content translated into multiple languages, ensure the default language loads immediately, then load other languages in a prioritized order based on the user's preferences or geographic location.

Best Practices for Using experimental_SuspenseList

• Keep it Simple: Don't overuse experimental_SuspenseList. Only use it when the order in which content is revealed significantly impacts the user experience.
• Optimize Data Fetching: experimental_SuspenseList only controls the reveal order, not the actual data fetching. Ensure that your data fetching is efficient to minimize loading times. Use techniques like memoization and caching to avoid unnecessary re-fetches.
• Provide Meaningful Fallbacks: The fallback prop of the <Suspense> component is crucial. Provide clear and informative loading indicators to let users know that content is on its way. Consider using skeleton loaders for a more visually appealing loading experience.
• Test Thoroughly: Test your loading states in different network conditions to ensure that the user experience is acceptable even with slow connections.
• Consider Accessibility: Ensure that your loading indicators are accessible to users with disabilities. Use ARIA attributes to provide semantic information about the loading process.
• Monitor Performance: Use browser developer tools to monitor the performance of your application and identify any bottlenecks in the loading process.
• Code Splitting: Combine Suspense with code splitting to load only the necessary components and data when they are needed.
• Avoid Over-Nesting: Deeply nested Suspense boundaries can lead to complex loading behavior. Keep the component tree relatively flat to simplify debugging and maintenance.
• Graceful Degradation: Consider how your application will behave if JavaScript is disabled or if there are errors during data fetching. Provide alternative content or error messages to ensure a usable experience.

Limitations and Considerations

• Experimental Status: experimental_SuspenseList is still an experimental API, which means it is subject to change or removal in future React releases. Use it with caution and be prepared to adapt your code as the API evolves.
• Complexity: While experimental_SuspenseList provides powerful control over loading states, it can also add complexity to your code. Carefully consider whether the benefits outweigh the added complexity.
• React Concurrent Mode Required: experimental_SuspenseList and the use hook, require React Concurrent Mode to function correctly. Ensure your application is configured to use Concurrent Mode.
• Server-Side Rendering (SSR): Implementing Suspense with SSR can be more complex than client-side rendering. You need to ensure that the server waits for the data to resolve before sending the HTML to the client to avoid hydration mismatches.

Conclusion

experimental_SuspenseList is a valuable tool for crafting sophisticated and user-friendly loading experiences in React applications. By understanding its loading strategies and applying best practices, you can create interfaces that feel faster, more responsive, and less distracting. While it's still experimental, the concepts and techniques learned by using experimental_SuspenseList are invaluable and will likely influence future React APIs for managing asynchronous data and UI updates. As React continues to evolve, mastering Suspense and related features will become increasingly important for building high-quality web applications for a global audience. Remember to always prioritize the user experience and choose the loading strategy that best suits the specific needs of your application. Experiment, test, and iterate to create the best possible loading experience for your users.