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docs: use function bindings in "when not to use effect" #15544

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42 changes: 7 additions & 35 deletions documentation/docs/02-runes/04-$effect.md
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Expand Up @@ -280,62 +280,34 @@ You might be tempted to do something convoluted with effects to link one value t
</label>
```

Instead, use callbacks where possible ([demo](/playground/untitled#H4sIAAAAAAAACo1SMW6EMBD8imWluFMSIEUaDiKlvy5lSOHjlhOSMRZeTiDkv8deMEEJRcqdmZ1ZjzzxqpZgePo5cRw18JQA_sSVaPz0rnVk7iDRYxdhYA8vW4Wg0NnwzJRdrfGtUAVKQIYtCsly9pIkp4AZ7cQOezAoEA7JcWUkVBuCdol0dNWrEutWsV5fHfnhPQ5wZJMnCwyejxCh6G6A0V3IHk4zu_jOxzzPBxBld83PTr7xXrb3rUNw8PbiYJ3FP22oTIoLSComq5XuXTeu8LzgnVA3KDgj13wiQ8taRaJ82rzXskYM-URRlsXktejjgNLoo9e4fyf70_8EnwncySX1GuunX6kGRwnzR_BgaPNaGy3FmLJKwrCUeBM6ZUn0Cs2mOlp3vwthQJ5i14P9st9vZqQlsQIAAA==)):
Instead, use `oninput` callbacks or — better still — [function bindings](bind#Function-bindings) where possible ([demo](/playground/untitled#H4sIAAAAAAAAE51SsW6DMBT8FcvqABINdOhCIFKXTt06lg4GHpElYyz8iECIf69tcIIipo6-u3f3fPZMJWuBpvRzkBXyTpKSy5rLq6YRbbgATdOfmeKkrMgCBt9GPpQ66RsItFjJNBzhVScRJBobmumq5wovhSxQABLskAmSk7ckOXtMKyM22ItGhhAk4Z0R0OwIN-tIQzd-90HVhvy2HsGNiQFCMltBgd7XoecV2xzXNV7XaEcth7ZfRv7kujnsTX2Qd7USb5rFjwZkJlgJwpWRcakG04cpOS9oz-QVCuoeInXW-RyEJL-sG0b7Wy6kZWM-u7CFxM5tdrIl9qg72vB74H-y7T2iXROHyVb0CLanp1yNk4D1A1jQ91hzrQSbUtIIGLcir0ylJDm9Q7urz42bX4UwIk2xH2D5Xf4A7SeMcMQCAAA=)):

```svelte
<script>
let total = 100;
let spent = $state(0);
let left = $state(total);

function updateSpent(e) {
spent = +e.target.value;
function updateSpent(value) {
spent = value;
left = total - spent;
}

function updateLeft(e) {
left = +e.target.value;
function updateLeft(value) {
left = value;
spent = total - left;
}
</script>

<label>
<input type="range" value={spent} oninput={updateSpent} max={total} />
<input type="range" bind:value={() => spent, updateSpent} max={total} />
{spent}/{total} spent
</label>

<label>
<input type="range" value={left} oninput={updateLeft} max={total} />
<input type="range" bind:value={() => left, updateLeft} max={total} />
{left}/{total} left
</label>
```

If you need to use bindings, for whatever reason (for example when you want some kind of "writable `$derived`"), consider using getters and setters to synchronise state ([demo](/playground/untitled#H4sIAAAAAAAACpWRwW6DMBBEf8WyekikFOihFwcq9TvqHkyyQUjGsfCCQMj_XnvBNKpy6Qn2DTOD1wu_tRocF18Lx9kCFwT4iRvVxenT2syNoDGyWjl4xi93g2AwxPDSXfrW4oc0EjUgwzsqzSr2VhTnxJwNHwf24lAhHIpjVDZNwy1KS5wlNoGMSg9wOCYksQccerMlv65p51X0p_Xpdt_4YEy9yTkmV3z4MJT579-bUqsaNB2kbI0dwlnCgirJe2UakJzVrbkKaqkWivasU1O1ULxnOVk3JU-Uxti0p_-vKO4no_enbQ_yXhnZn0aHs4b1jiJMK7q2zmo1C3bTMG3LaZQVrMjeoSPgaUtkDxePMCEX2Ie6b_8D4WyJJEwCAAA=)):

```svelte
<script>
let total = 100;
let spent = $state(0);

let left = {
get value() {
return total - spent;
},
set value(v) {
spent = total - v;
}
};
</script>

<label>
<input type="range" bind:value={spent} max={total} />
{spent}/{total} spent
</label>

<label>
<input type="range" bind:value={left.value} max={total} />
{left.value}/{total} left
</label>
```

If you absolutely have to update `$state` within an effect and run into an infinite loop because you read and write to the same `$state`, use [untrack](svelte#untrack).