$props
The inputs to a component are referred to as props, which is short for properties. You pass props to components just like you pass attributes to elements:
<script>
import MyComponent from './MyComponent.svelte';
</script>
<MyComponent adjective="cool" />On the other side, inside MyComponent.svelte, we can receive props with the $props rune...
<script>
let props = $props();
</script>
<p>this component is {props.adjective}</p>...though more commonly, you’ll destructure your props:
<script>
let { adjective } = $props();
</script>
<p>this component is {adjective}</p>Fallback values
Destructuring allows us to declare fallback values, which are used if the parent component does not set a given prop:
let { let adjective: anyadjective = 'happy' } = function $props(): anyDeclares the props that a component accepts. Example:
let { optionalProp = 42, requiredProp, bindableProp = $bindable() }: { optionalProp?: number; requiredProps: string; bindableProp: boolean } = $props();
$props();Fallback values are not turned into reactive state proxies (see Updating props for more info)
Renaming props
We can also use the destructuring assignment to rename props, which is necessary if they’re invalid identifiers, or a JavaScript keyword like super:
let { super: let trouper: anytrouper = 'lights are gonna find me' } = function $props(): anyDeclares the props that a component accepts. Example:
let { optionalProp = 42, requiredProp, bindableProp = $bindable() }: { optionalProp?: number; requiredProps: string; bindableProp: boolean } = $props();
$props();Rest props
Finally, we can use a rest property to get, well, the rest of the props:
let { let a: anya, let b: anyb, let c: anyc, ...let others: anyothers } = function $props(): anyDeclares the props that a component accepts. Example:
let { optionalProp = 42, requiredProp, bindableProp = $bindable() }: { optionalProp?: number; requiredProps: string; bindableProp: boolean } = $props();
$props();Updating props
References to a prop inside a component update when the prop itself updates — when count changes in App.svelte, it will also change inside Child.svelte. But the child component is able to temporarily override the prop value, which can be useful for unsaved ephemeral state (demo):
<script>
import Child from './Child.svelte';
let count = $state(0);
</script>
<button onclick={() => (count += 1)}>
clicks (parent): {count}
</button>
<Child {count} /><script>
let { count } = $props();
</script>
<button onclick={() => (count += 1)}>
clicks (child): {count}
</button>Prop variables are not automatically deeply reactive. What happens when mutating one of their properties depends on what the parent passed in. For example if the parent passed a non-reactive POJO as a prop, setting a property of that object in the child will not cause the component to update (demo):
<script>
import Child from './Child.svelte';
let object = $state({count: 0});
</script>
<Child {object} /><script>
let { object } = $props();
</script>
<button onclick={() => {
// has no effect
object.count += 1
}}>
clicks: {object.count}
</button>However if the value passed in by the parent component is itself a deeply reactive state object, then it will be deeply reactive in the child, too (demo):
<script>
import Child from './Child.svelte';
let object = $state({count: 0});
</script>
<Child {object} /><script>
let { object } = $props();
</script>
<button onclick={() => {
// will cause the count below to update
object.count += 1
}}>
clicks: {object.count}
</button>The fallback value of a prop not declared with $bindable is treated like a non-reactive POJO, and therefore also doesn’t update the component when mutating its properties.
<script>
let { object = { count = 0 } } = $props();
</script>
<button onclick={() => {
// has no effect if the fallback value is used
object.count += 1
}}>
clicks: {object.count}
</button>In general, mutating props is discouraged, instead use callback props to make it easier to reason about state and changes to that state. If parent and child should share (and be allowed to mutate) the same object, then use the $bindable rune.
Type safety
You can add type safety to your components by annotating your props, as you would with any other variable declaration. In TypeScript that might look like this...
<script lang="ts">
let { adjective }: { adjective: string } = $props();
</script>...while in JSDoc you can do this:
<script>
/** @type {{ adjective: string }} */
let { adjective } = $props();
</script>You can, of course, separate the type declaration from the annotation:
<script lang="ts">
interface Props {
adjective: string;
}
let { adjective }: Props = $props();
</script>Adding types is recommended, as it ensures that people using your component can easily discover which props they should provide.