Svelte

Avoid common Svelte mistakes — reactivity triggers, store subscriptions, and SvelteKit SSR gotchas.

Reactivity Triggers

• Assignment triggers reactivity — arr = arr after push, or use arr = [...arr, item]
• Array methods don't trigger — arr.push() needs reassignment: arr = arr
• Object mutation same issue — obj.key = val; obj = obj or spread: obj = {...obj, key: val}
• $: reactive statements run on dependency change — but only top-level assignments tracked

Reactive Statements

• $: runs when dependencies change — list all dependencies used
• $: { } block for multiple statements — all run together
• $: order matters — later statements can depend on earlier
• Avoid side effects in $: — prefer derived values, use onMount for effects

Stores

• $store auto-subscribes in component — automatic unsubscribe on destroy
• Manual subscribe needs unsubscribe — const unsub = store.subscribe(v => ...); onDestroy(unsub)
• writable for read/write — readable for external data sources
• derived for computed values — derived(store, $s => $s * 2)

Component Lifecycle

• onMount runs after first render — return cleanup function
• No access to DOM before onMount — document etc. not available in SSR
• beforeUpdate / afterUpdate for DOM sync — rarely needed
• tick() to wait for DOM update — await tick() after state change

Props

• export let propName to declare — required by default
• export let propName = default for optional — default value if not passed
• Props are reactive — component re-renders on change
• $$props and $$restProps for pass-through — but explicit props preferred

Events

• createEventDispatcher for custom events — dispatch('eventName', data)
• on:eventName to listen — on:click, on:customEvent
• on:click|preventDefault modifiers — |stopPropagation, |once
• Event forwarding: on:click without handler — forwards to parent

SvelteKit

• +page.svelte for pages — +page.server.ts for server-only load
• load function for data fetching — runs on server and client navigation
• $app/stores for page, navigating, etc. — $page.params, $page.url
• form actions for mutations — progressive enhancement, works without JS

SSR Gotchas

• browser from $app/environment — check before using window/document
• onMount only runs client-side — safe for browser APIs
• Stores initialized on server shared between requests — use context for request-specific
• fetch in load is special — relative URLs work, credentials handled

Svelte 5 Runes

• $state() replaces let for reactivity — let count = $state(0)
• $derived replaces $: for computed — let doubled = $derived(count * 2)
• $effect for side effects — replaces $: with side effects
• Runes are opt-in per file — can mix with Svelte 4 syntax

Common Mistakes

• Destructuring props loses reactivity — let { prop } = $props() in Svelte 5, or don't destructure in 4
• Store value vs store — $store for value, store for subscribe/set
• Transition on conditional — {#if show}<div transition:fade> not on wrapper
• Key block for re-render — {#key value}...{/key} destroys and recreates