SvelteMisc

TypeScript

You can use TypeScript within Svelte components. IDE extensions like the Svelte VS Code extension will help you catch errors right in your editor, and svelte-check does the same on the command line, which you can integrate into your CI.

<script lang="ts">

To use TypeScript inside your Svelte components, add lang="ts" to your script tags:

<script lang="ts">
	let name: string = 'world';

	function greet(name: string) {
		alert(`Hello, ${name}!`);
	}
</script>

<button onclick={(e: Event) => greet(e.target.innerText)}>
	{name as string}
</button>

Doing so allows you to use TypeScript’s type-only features. That is, all features that just disappear when transpiling to JavaScript, such as type annotations or interface declarations. Features that require the TypeScript compiler to output actual code are not supported. This includes:

using enums
using private, protected or public modifiers in constructor functions together with initializers
using features that are not yet part of the ECMAScript standard (i.e. not level 4 in the TC39 process) and therefore not implemented yet within Acorn, the parser we use for parsing JavaScript

If you want to use one of these features, you need to setup up a script preprocessor.

Preprocessor setup

To use non-type-only TypeScript features within Svelte components, you need to add a preprocessor that will turn TypeScript into JavaScript.

Using SvelteKit or Vite

The easiest way to get started is scaffolding a new SvelteKit project by typing npx sv create, following the prompts and choosing the TypeScript option.

svelte.config

import { import vitePreprocessvitePreprocess } from '@sveltejs/kit/vite';

const const config: {
    preprocess: any;
}config = {
	preprocess: anypreprocess: import vitePreprocessvitePreprocess()
};

export default const config: {
    preprocess: any;
}config;

If you don’t need or want all the features SvelteKit has to offer, you can scaffold a Svelte-flavoured Vite project instead by typing npm create vite@latest and selecting the svelte-ts option.

svelte.config

import { function vitePreprocess(opts?: VitePreprocessOptions | undefined): import("svelte/compiler").PreprocessorGroupvitePreprocess } from '@sveltejs/vite-plugin-svelte';

const const config: {
    preprocess: PreprocessorGroup;
}config = {
	preprocess: PreprocessorGrouppreprocess: function vitePreprocess(opts?: VitePreprocessOptions | undefined): import("svelte/compiler").PreprocessorGroupvitePreprocess()
};

export default const config: {
    preprocess: PreprocessorGroup;
}config;

In both cases, a svelte.config.js with vitePreprocess will be added. Vite/SvelteKit will read from this config file.

Other build tools

If you’re using tools like Rollup or Webpack instead, install their respective Svelte plugins. For Rollup that’s rollup-plugin-svelte and for Webpack that’s svelte-loader. For both, you need to install typescript and svelte-preprocess and add the preprocessor to the plugin config (see the respective READMEs for more info). If you’re starting a new project, you can also use the rollup or webpack template to scaffold the setup from a script.

If you’re starting a new project, we recommend using SvelteKit or Vite instead

Typing $props

Type $props just like a regular object with certain properties.

<script lang="ts">
	import type { Snippet } from 'svelte';

	interface Props {
		requiredProperty: number;
		optionalProperty?: boolean;
		snippetWithStringArgument: Snippet<[string]>;
		eventHandler: (arg: string) => void;
		[key: string]: unknown;
	}

	let {
		requiredProperty,
		optionalProperty,
		snippetWithStringArgument,
		eventHandler,
		...everythingElse
	}: Props = $props();
</script>

<button onclick={() => eventHandler('clicked button')}>
	{@render snippetWithStringArgument('hello')}
</button>

Generic $props

Components can declare a generic relationship between their properties. One example is a generic list component that receives a list of items and a callback property that receives an item from the list. To declare that the items property and the select callback operate on the same types, add the generics attribute to the script tag:

<script lang="ts" generics="Item extends { text: string }">
	interface Props {
		items: Item[];
		select(item: Item): void;
	}

	let { items, select }: Props = $props();
</script>

{#each items as item}
	<button onclick={() => select(item)}>
		{item.text}
	</button>
{/each}

The content of generics is what you would put between the <...> tags of a generic function. In other words, you can use multiple generics, extends and fallback types.

Typing wrapper components

In case you’re writing a component that wraps a native element, you may want to expose all the attributes of underlying element to the user. In that case, use (or extend from) one of the interfaces provided by svelte/elements. Here’s an example for a Button component:

<script lang="ts">
	import type { HTMLButtonAttributes } from 'svelte/elements';

	let { children, ...rest }: HTMLButtonAttributes = $props();
</script>

<button {...rest}>
	{@render children()}
</button>

Typing $state

You can type $state like any other variable.

let let count: numbercount: number = function $state<0>(initial: 0): 0 (+1 overload)
namespace $stateDeclares reactive state.
Example:
let count = $state(0);
https://svelte-5-preview.vercel.app/docs/runes#$state
@paraminitial The initial value$state(0);

If you don’t give $state an initial value, part of its types will be undefined.

// Error: Type 'number | undefined' is not assignable to type 'number'
let let count: numbercount: number = function $state<number>(): number | undefined (+1 overload)
namespace $stateDeclares reactive state.
Example:
let count = $state(0);
https://svelte-5-preview.vercel.app/docs/runes#$state
@paraminitial The initial value$state();

If you know that the variable will be defined before you first use it, use an as casting. This is especially useful in the context of classes:

class class CounterCounter {
	Counter.count: numbercount = function $state<number>(): number | undefined (+1 overload)
namespace $stateDeclares reactive state.
Example:
let count = $state(0);
https://svelte-5-preview.vercel.app/docs/runes#$state
@paraminitial The initial value$state() as number;
	constructor(initial: numberinitial: number) {
		this.Counter.count: numbercount = initial: numberinitial;
	}
}

The Component type

Svelte components are of type Component. You can use it and its related types to express a variety of constraints.

Using it together with dynamic components to restrict what kinds of component can be passed to it:

<script lang="ts">
	import type { Component } from 'svelte';

	interface Props {
		// only components that have at most the "prop"
		// property required can be passed
		DynamicComponent: Component<{ prop: string }>;
	}

	let { DynamicComponent }: Props = $props();
</script>

<DynamicComponent prop="foo" />

Closely related to the Component type is the ComponentProps type which extracts the properties a component expects.

import type { interface Component<Props extends Record<string, any> = {}, Exports extends Record<string, any> = {}, Bindings extends keyof Props | "" = string>Can be used to create strongly typed Svelte components.
Example:
You have component library on npm called component-library, from which
you export a component called MyComponent. For Svelte+TypeScript users,
you want to provide typings. Therefore you create a index.d.ts:
import type { Component } from 'svelte';
export declare const MyComponent: Component&#x3C;{ foo: string }> {}
Typing this makes it possible for IDEs like VS Code with the Svelte extension
to provide intellisense and to use the component like this in a Svelte file
with TypeScript:
&#x3C;script lang="ts">
	import { MyComponent } from "component-library";
&#x3C;/script>
&#x3C;MyComponent foo={'bar'} />Component, type ComponentProps<Comp extends SvelteComponent | Component<any, any>> = Comp extends SvelteComponent<infer Props extends Record<string, any>, any, any> ? Props : Comp extends Component<infer Props extends Record<...>, any, string> ? Props : neverConvenience type to get the props the given component expects.
Example: Ensure a variable contains the props expected by MyComponent:
import type { ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';

// Errors if these aren't the correct props expected by MyComponent.
const props: ComponentProps&#x3C;MyComponent> = { foo: 'bar' };
Example: A generic function that accepts some component and infers the type of its props:
import type { Component, ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';

function withProps&#x3C;TComponent extends Component&#x3C;any>>(
	component: TComponent,
	props: ComponentProps&#x3C;TComponent>
) {};

// Errors if the second argument is not the correct props expected by the component in the first argument.
withProps(MyComponent, { foo: 'bar' });ComponentProps } from 'svelte';
import type MyComponent = SvelteComponent<Record<string, any>, any, any>
const MyComponent: ComponentTypeMyComponent from './MyComponent.svelte';

function function withProps<TComponent extends Component<any>>(component: TComponent, props: ComponentProps<TComponent>): voidwithProps<function (type parameter) TComponent in withProps<TComponent extends Component<any>>(component: TComponent, props: ComponentProps<TComponent>): voidTComponent extends interface Component<Props extends Record<string, any> = {}, Exports extends Record<string, any> = {}, Bindings extends keyof Props | "" = string>Can be used to create strongly typed Svelte components.
Example:
You have component library on npm called component-library, from which
you export a component called MyComponent. For Svelte+TypeScript users,
you want to provide typings. Therefore you create a index.d.ts:
import type { Component } from 'svelte';
export declare const MyComponent: Component&#x3C;{ foo: string }> {}
Typing this makes it possible for IDEs like VS Code with the Svelte extension
to provide intellisense and to use the component like this in a Svelte file
with TypeScript:
&#x3C;script lang="ts">
	import { MyComponent } from "component-library";
&#x3C;/script>
&#x3C;MyComponent foo={'bar'} />Component<any>>(
	component: TComponent extends Component<any>component: function (type parameter) TComponent in withProps<TComponent extends Component<any>>(component: TComponent, props: ComponentProps<TComponent>): voidTComponent,
	props: ComponentProps<TComponent>props: type ComponentProps<Comp extends SvelteComponent | Component<any, any>> = Comp extends SvelteComponent<infer Props extends Record<string, any>, any, any> ? Props : Comp extends Component<infer Props extends Record<...>, any, string> ? Props : neverConvenience type to get the props the given component expects.
Example: Ensure a variable contains the props expected by MyComponent:
import type { ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';

// Errors if these aren't the correct props expected by MyComponent.
const props: ComponentProps&#x3C;MyComponent> = { foo: 'bar' };
Example: A generic function that accepts some component and infers the type of its props:
import type { Component, ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';

function withProps&#x3C;TComponent extends Component&#x3C;any>>(
	component: TComponent,
	props: ComponentProps&#x3C;TComponent>
) {};

// Errors if the second argument is not the correct props expected by the component in the first argument.
withProps(MyComponent, { foo: 'bar' });ComponentProps<function (type parameter) TComponent in withProps<TComponent extends Component<any>>(component: TComponent, props: ComponentProps<TComponent>): voidTComponent>
) {}

// Errors if the second argument is not the correct props expected
// by the component in the first argument.
function withProps<ComponentType>(component: ComponentType, props: Record<string, any>): voidwithProps(const MyComponent: ComponentTypeMyComponent, { foo: stringfoo: 'bar' });

Enhancing built-in DOM types

Svelte provides a best effort of all the HTML DOM types that exist. Sometimes you may want to use experimental attributes or custom events coming from an action. In these cases, TypeScript will throw a type error, saying that it does not know these types. If it’s a non-experimental standard attribute/event, this may very well be a missing typing from our HTML typings. In that case, you are welcome to open an issue and/or a PR fixing it.

In case this is a custom or experimental attribute/event, you can enhance the typings like this:

additional-svelte-typings.d

declare namespace svelteHTML {
	// enhance elements
	interface interface svelteHTML.IntrinsicElementsIntrinsicElements {
		'my-custom-element': { someattribute: stringsomeattribute: string; 'on:event': (e: CustomEvent<any>e: interface CustomEvent<T = any>MDN Reference
CustomEvent<any>) => void };
	}
	// enhance attributes
	interface interface svelteHTML.HTMLAttributes<T>HTMLAttributes<function (type parameter) T in HTMLAttributes<T>T> {
		// If you want to use the beforeinstallprompt event
		svelteHTML.HTMLAttributes<T>.onbeforeinstallprompt?: ((event: any) => any) | undefinedonbeforeinstallprompt?: (event: anyevent: any) => any;
		// If you want to use myCustomAttribute={..} (note: all lowercase)
		svelteHTML.HTMLAttributes<T>.mycustomattribute?: anymycustomattribute?: any; // You can replace any with something more specific if you like
	}
}

Then make sure that d.ts file is referenced in your tsconfig.json. If it reads something like "include": ["src/**/*"] and your d.ts file is inside src, it should work. You may need to reload for the changes to take effect.

You can also declare the typings by augmenting the svelte/elements module like this:

additional-svelte-typings.d

import { HTMLButtonAttributes } from 'svelte/elements';

declare module 'svelte/elements' {
	export interface SvelteHTMLElements {
		'custom-button': HTMLButtonAttributes;
	}

	// allows for more granular control over what element to add the typings to
	export interface HTMLButtonAttributes {
		HTMLButtonAttributes.veryexperimentalattribute?: string | undefinedveryexperimentalattribute?: string;
	}
}

export {}; // ensure this is not an ambient module, else types will be overridden instead of augmented

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