Svelte

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)