Svelte Technical Debt Cleanup

Dead Sapper Routes, Implicit Reactive Chains, and Leaking Store Subscriptions

If you’re still on Sapper, that’s the obvious debt. Sapper is unmaintained and its routing, data loading, and build system are all replaced by SvelteKit’s superior implementations. But even SvelteKit apps accumulate debt.

The most common: reactive statement abuse. The $: syntax is powerful but implicit. Complex derived state chains become hard to trace - you change one variable and five reactive statements fire in an order that nobody fully understands. Store subscriptions leak when components unmount incorrectly. Large components with dozens of reactive declarations become fragile.

SvelteKit Routing Rewrite and Incremental Runes Adoption

For Sapper to SvelteKit migrations, we rewrite the routing layer using SvelteKit’s file-based conventions, convert data loading to load functions, and migrate the build configuration. The components themselves usually transfer with minimal changes.

For Svelte 5, we adopt runes incrementally. $state() replaces let for reactive variables. $derived() replaces $: for computed values. $effect() replaces reactive statements that produce side effects. Each migration makes reactivity explicit and traceable - no more guessing which $: block triggered which update.

Predictable Reactivity, Smaller Bundles, and God Components Decomposed

Reactivity becomes predictable. Svelte 5’s runes make state changes explicit - you can trace exactly what’s reactive, what’s derived, and what produces side effects. Debugging reactive chains goes from detective work to reading code.

Bundle sizes shrink because SvelteKit’s build system tree-shakes more aggressively than Sapper’s. Load times improve because data fetching moves to server-side load functions that cache properly. Your app feels snappier because unnecessary client-side work moves to the server.

Component architecture also improves during the migration. Legacy Svelte codebases tend to accumulate “god components” - files with 500+ lines mixing data fetching, state management, DOM manipulation, and business logic in a single .svelte file. We decompose these into focused units: presentational components that accept props, logic encapsulated in .svelte.js modules using runes, and layout components that handle composition. The $props() rune replaces the old export let pattern with a cleaner destructuring syntax that supports defaults and rest props natively. Snippet blocks replace the awkward slot-based composition model, making component APIs more readable and type-safe. TypeScript integration tightens as well - SvelteKit’s generated types for load functions, form actions, and page data eliminate an entire class of runtime errors that plagued Sapper applications.

Runes-Only Lint Rules, Component Size Limits, and Declarative Adapter Deploys

We configure eslint-plugin-svelte with rules that enforce runes syntax and prevent falling back to legacy patterns. Component size limits encourage extraction into smaller, focused components.

SvelteKit’s adapters standardize deployment. No more custom build scripts that one developer wrote and nobody else understands. The adapter configuration is declarative and version-controlled. Upgrading Svelte versions becomes a package update, not a project.