svelte

Run npx sv migrate svelte-5 on a clean git tree, then resolve what the script leaves behind: convert $: assignment blocks to $derived rather than $effect, replace createEventDispatcher with callback props, turn slots into snippets rendered with {@render children()}, and set compilerOptions.runes last so leftovers become compile errors.

The usual causes are dependency tracking traps: optional chaining like chart?.update(scheme) never registers scheme when chart is nullish, state read after an await falls outside the tracking scope, and destructuring snapshots a value. Read every dependency unconditionally before branching or awaiting.

A universal load in +page.js runs in both the browser and on the server, so it cannot import server-only modules and must guard window or localStorage with browser from $app/environment. A server load in +page.server.js runs only on the server and can read secrets, but its return value is serialized into the HTML.

No, SvelteKit uses devalue, so Date, Map, Set, BigInt, RegExp, and cyclical references cross the boundary intact without stringifying. Class instances are the exception: they need a transport transporter registered in hooks, or should be flattened to plain data.

A server process is shared by every visitor, so a mutable let at module scope in +page.server.js or a .svelte.js module leaks one user's data to the next. Per-user state belongs in a cookie plus database read through event.locals, or in context scoped to the request's component tree.

Form actions are stable and feature-complete, so default to them for existing projects. Remote functions (SvelteKit 2.27+) are still experimental and require opting into kit.experimental.remoteFunctions plus compilerOptions.experimental.async, which changes effect ordering app-wide, so recommend them only for new projects that accept API churn.