`onHasValue`

The onHasValue function registers a listener that is called whenever a specified Value is added to or removed from the Store.

onHasValue(
  valueId: MaybeGetter<IdOrNull>,
  listener: HasValueListener,
  mutator?: boolean,
  storeOrStoreId?: MaybeGetter<undefined | StoreOrStoreId>,
): void

	Type	Description
`valueId`	`MaybeGetter<IdOrNull>`	The `Id` of the `Value` to listen to, or `null` to listen to any `Value`.
`listener`	`HasValueListener`	The function to call when the `Value` changes.
`mutator?`	`boolean`	An optional boolean indicating the listener mutates `Store` data.
`storeOrStoreId?`	`MaybeGetter<undefined \| StoreOrStoreId>`	The `Store` to use, or its `Id`.
returns	`void`	This has no return value.

Example

This example registers a Svelte listener and responds to a TinyBase change.

App.svelte

<svelte:options runes={true} />

<script>
  import {onHasValue} from 'tinybase/ui-svelte';

  let {store} = $props();
  let seen = $state('');

  onHasValue('open', () => (seen = 'changed'), false, store);
</script>

{seen}

import {flushSync, mount} from 'svelte';
import {createStore} from 'tinybase';
import App from './App.svelte';

const store = createStore()
  .setTables({
    pets: {
      fido: {species: 'dog', color: 'brown', sold: false, next: 'felix'},
      felix: {species: 'cat', color: 'black', sold: true},
    },
    species: {dog: {price: 5}, cat: {price: 4}},
  })
  .setValues({open: true, employees: 3});
const app = document.body.appendChild(document.createElement('div'));
flushSync(() => mount(App, {target: app, props: {store}}));
flushSync(() => {
  store.delValue('open');
});
flushSync();
console.log(app.textContent);
// -> 'changed'

Since

v8.1.0