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| $bindable |
Ordinarily, props go one way, from parent to child. This makes it easy to understand how data flows around your app.
In Svelte, component props can be bound, which means that data can also flow up from child to parent. This isn't something you should do often, but it can simplify your code if used sparingly and carefully.
It also means that a state proxy can be mutated in the child.
[!NOTE] Mutation is also possible with normal props, but is strongly discouraged — Svelte will warn you if it detects that a component is mutating state it does not 'own'.
To mark a prop as bindable, we use the $bindable rune:
/// file: FancyInput.svelte
<script>
let { value = $bindable(), ...props } = $props();
</script>
<input bind:value={value} {...props} />
<style>
input {
font-family: 'Comic Sans MS';
color: deeppink;
}
</style>
Now, a component that uses <FancyInput> can add the bind: directive (demo):
/// App.svelte
<script>
import FancyInput from './FancyInput.svelte';
let message = $state('hello');
</script>
<FancyInput bind:value={message} />
<p>{message}</p>
The parent component doesn't have to use bind: — it can just pass a normal prop. Some parents don't want to listen to what their children have to say.
In this case, you can specify a fallback value for when no prop is passed at all:
/// file: FancyInput.svelte
let { value = $bindable('fallback'), ...props } = $props();