Efficient Lazy Loading in Svelte: A Practical Guide for Svelte 4 and Svelte 5 (Runes)

Modern web applications are growing in complexity and size, often including many components that are not needed on the initial page load. Lazy loading is a powerful technique to optimize bundle size and improve performance by loading components only when they are actually needed—such as widgets, modals, or rarely-used UI elements. In this post, we’ll explore how to implement lazy loading in Svelte, compare the syntax between Svelte 4 and Svelte 5 (runes mode), and discuss the benefits and potential drawbacks of this approach.

Why Lazy Load Components?

Performance: By splitting your bundle and loading code only when required, you reduce the initial load time and memory usage.
User Experience: Faster initial load means a snappier experience, especially on mobile devices or slow networks.
Scalability: As your app grows, lazy loading keeps it maintainable and modular, making it easier to manage and extend.

Svelte 4 Syntax

In Svelte 4, dynamic imports are combined with the <svelte:component> tag for lazy loading:


{#await import('./MyWidget.svelte') then Widget}
  <svelte:component this={Widget.default} someProp={value} />
{/await}

<svelte:component> allows you to render a component dynamically.
Props are passed as usual.

Svelte 5 (Runes) Syntax

Svelte 5 introduces runes and direct component invocation, but for lazy loading, the recommended approach is to use the imported component as a tag inside an {#await} block:


{#await import('./MyWidget.svelte') then Widget}
  <Widget.default someProp={value} />
{/await}

This is the idiomatic way for lazy loading in Svelte 5.
Direct invocation (Widget.default({ someProp: value })) is for advanced use cases, not markup.

Dynamic Import in a Loop

You can dynamically load a component inside an {#each} loop, which is useful for rendering lists of widgets or cards:


{#each items as item}
  {#await import('./MyWidget.svelte') then Widget}
    <Widget.default data={item} />
  {/await}
{/each}

Performance Note:

Svelte will only import the module once, even if the loop has many items. Subsequent imports use the cached module, so there is no repeated network or disk load.
The only overhead is rendering multiple component instances, which is normal for any list rendering.

Drawbacks and Considerations

Initial Delay: The first time a component is loaded, there may be a slight delay as the code is fetched and parsed. For critical UI, consider preloading or keeping it in the main bundle.
SSR Compatibility: Lazy loading is primarily a client-side optimization. If you use server-side rendering, ensure your approach is compatible or fallback gracefully.
Code Splitting: Too many small chunks can increase HTTP requests. Group related components when possible.
State Management: If your lazy-loaded component depends on global state, ensure the state is available when the component mounts.

Conclusion

Lazy loading components in Svelte is a simple yet effective way to optimize your app’s performance and scalability. By leveraging dynamic imports and Svelte’s template syntax, you can keep your initial bundle small and load features only when needed. Both Svelte 4 and Svelte 5 support this pattern, with Svelte 5 offering a more streamlined syntax.

References:

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Richard Fu @furic