Code Splitting

Delivering high-speed and responsive web applications has become an important aspect of modern web and mobile development. However, developers often bundle their JavaScript code into a single file when building web applications. While this approach simplifies deployment, it can lead to significant performance issues. Users end up downloading, parsing, and executing large amounts of JavaScript code, even if they only need a small portion for their current interaction.

This leads to:

  • Slower page loads
  • Delayed interactivity
  • Poor user experience

This is where code splitting comes into play.

What is Code Splitting?

Code splitting is a technique for dividing JavaScript code into smaller bundles that can be loaded independently and in parallel. Instead of forcing the users to download the entire application upfront, we can deliver only the code needed for the current page or functionality. This approach can significantly improve your application’s initial load time and overall performance.

Modern tools like webpack can be used for code splitting. Webpack can define entry points for an application and generate separate bundles. It serves as a build tool, ensuring that only the required parts of the application are sent to the client when needed, improving efficiency and user experience.

Code Splitting Techniques

There are three main techniques for implementing code splitting:

1. Multiple entry points

The most straightforward approach is splitting code through multiple entry points in your build configuration. While this method is intuitive, it requires manual configuration and can lead to code duplication if not carefully managed. It’s best suited for applications with clearly defined, separate sections.

2. Dynamic imports

The recommended and most flexible approach is using dynamic imports. This method allows you to split code at the component or functionality level.

async function loadFeature() {
  const { feature } = await import('./feature.js');
  feature.start();
}
// Load the feature only when needed
button.addEventListener('click', loadFeature);

This approach is particularly powerful because it enables:

  • Loading code on demand.
  • Conditional loading based on user interactions or system requirements.
  • Better resource management.

3. Prevention of duplication

Modern bundlers like webpack provide tools such as SplitChunksPlugin to automatically identify and extract common dependencies into shared bundles. This ensures that frequently used code is cached effectively and not duplicated across different chunks.

Benefits of Code Splitting

Code splitting offers several key performance advantages:

  • Reduced initial bundle size, leading to faster page loads.
  • Improved caching efficiency as updates to one part of the application don’t invalidate the entire bundle.
  • Better resource utilization as users only download the code they need.
  • Reduced parsing and compilation time during startup.

Best Practices of Code Splitting

To make the most of code splitting:

  • Analyze your bundles: Use tools like webpack-bundle-analyzer to understand your bundle composition and identify splitting opportunities.
  • Strategic splitting: Focus on splitting out large libraries and features that aren’t immediately needed.
  • Implement prefetching: Use resource hints like prefetch
    import(/* webpackPrefetch: true */ './future-feature.js');
  • Monitor performance: Keep track of key metrics like total blocking time (TBT) and interaction to next paint (INP) to ensure your splitting strategy is effective.
  • Use bundlers wisely: Configure your bundler to optimize chunk sizes and leverage features like automatic code splitting.

Challenges and Solutions

When implementing code splitting across different resource types, developers often face several challenges:

Bundle size management

  • Regular monitoring of bundle sizes.
  • Setting appropriate size thresholds.
  • Implementing automated size budgets.

Loading strategy

  • Determining optimal splitting points.
  • Managing loading priorities.
  • Handling loading failures gracefully.

Caching considerations

  • Implementing effective cache strategies.
  • Managing cache invalidation.
  • Optimizing cache hit rates.

Beyond JavaScript: A Comprehensive Approach

While code splitting is predominantly associated with JavaScript code splitting, modern web development demands a broader perspective. The technique can be effectively applied to various resource types:

CSS splitting

Breaking down large stylesheets into smaller, feature-specific chunks can significantly improve initial page load times. Using tools like mini-css-extract-plugin, you can:

  • Split CSS based on component usage.
  • Load styles dynamically with components.
  • Separate critical CSS from non-critical styles.

Asset optimization

Images, media files, and other assets can benefit from intelligent splitting strategies:

  • Implement lazy loading for images below the fold.
  • Use responsive loading techniques for different viewport sizes.
  • Split large sprite sheets into context-specific bundles.

WebAssembly and web workers

For computation-heavy applications:

  • Load WebAssembly modules on demand.
  • Split worker scripts into task-specific chunks.
  • Optimize performance-critical code paths.

Looking Ahead

Code splitting has moved from just an optimization technique to something far more crucial for web development. As applications grow and users demand ever-increasing performance, code-splitting strategies will be indispensable for optimal user experiences.

Remember that code splitting isn’t the only solution to increase performance. The best approach depends on your application’s specific needs, target audience, and performance requirements. Regular analysis and adjustment of your splitting strategy will help ensure your application maintains peak performance as it evolves.