Misc

TypeScript

You can use TypeScript within Svelte components. IDE extensions like the Svelte VSCode 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.

Setuppermalink

To use TypeScript within Svelte components, you need to add a preprocessor that will turn TypeScript into JavaScript.

Using SvelteKit or Vitepermalink

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

svelte.config.js

ts
import { vitePreprocess } from '@sveltejs/kit/vite';

const config = {
	preprocess: vitePreprocess()
};

export default 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.js

ts
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';

const config = {
	preprocess: vitePreprocess()
};

export default config;

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

Other build toolspermalink

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

<script lang="ts">permalink

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>

Propspermalink

Props can be typed directly on the export let statement:

<script lang="ts">
	export let name: string;
</script>

Slotspermalink

Slot and slot prop types are inferred from the types of the slot props passed to them:

<script lang="ts">
	export let name: string;
</script>

<slot {name} />

<!-- Later -->
<Comp let:name>
	<!--    ^ Inferred as string -->
	{name}
</Comp>

Eventspermalink

Events can be typed with createEventDispatcher:

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

	const dispatch = createEventDispatcher<{
		event: null; // does not accept a payload
		click: string; // has a required string payload
		type: string | null; // has an optional string payload
	}>();

	function handleClick() {
		dispatch('event');
		dispatch('click', 'hello');
	}

	function handleType() {
		dispatch('event');
		dispatch('type', Math.random() > 0.5 ? 'world' : null);
	}
</script>

<button on:click={handleClick} on:keydown={handleType}>Click</button>

Enhancing built-in DOM typespermalink

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.ts

ts
declare namespace svelteHTML {
	// enhance elements
	interface IntrinsicElements {
		'my-custom-element': { someattribute: string; 'on:event': (e: CustomEvent<any>) => void };
	}
	// enhance attributes
	interface HTMLAttributes<T> {
		// If you want to use on:beforeinstallprompt
		'on:beforeinstallprompt'?: (event: any) => any;
		// If you want to use myCustomAttribute={..} (note: all lowercase)
		mycustomattribute?: 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.

Since Svelte version 4.2 / svelte-check version 3.5 / VS Code extension version 107.10.0 you can also declare the typings by augmenting the svelte/elements module like this:

additional-svelte-typings.d.ts

ts
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 {
		veryexperimentalattribute?: string;
	}
}

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

Experimental advanced typingspermalink

A few features are missing from taking full advantage of TypeScript in more advanced use cases like typing that a component implements a certain interface, explicitly typing slots, or using generics. These things are possible using experimental advanced type capabilities. See this RFC for more information on how to make use of them.

The API is experimental and may change at any point

Limitationspermalink

No TS in markuppermalink

You cannot use TypeScript in your template's markup. For example, the following does not work:

<script lang="ts">
	let count = 10;
</script>

<h1>Count as string: {count as string}!</h1> <!-- ❌ Does not work -->
{#if count > 4}
	{@const countString: string = count} <!-- ❌ Does not work -->
	{countString}
{/if}

Reactive Declarationspermalink

You cannot type your reactive declarations with TypeScript in the way you type a variable. For example, the following does not work:

<script lang="ts">
	let count = 0;

	$: doubled: number = count * 2; // ❌ Does not work
</script>

You cannot add a : TYPE because it's invalid syntax in this position. Instead, you can move the definition to a let statement just above:

<script lang="ts">
	let count = 0;

	let doubled: number;
	$: doubled = count * 2;
</script>

Typespermalink

ComponentConstructorOptionspermalink

ts
interface ComponentConstructorOptions<
	Props extends Record<string, any> = Record<string, any>
> {…}

ts
target: Element | Document | ShadowRoot;

ts
anchor?: Element;

ts
props?: Props;

ts
context?: Map<any, any>;

ts
hydrate?: boolean;

ts
intro?: boolean;

ts
$$inline?: boolean;

ComponentEventspermalink

Convenience type to get the events the given component expects. Example:

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

   function handleCloseEvent(event: ComponentEvents<Component>['close']) {
	  console.log(event.detail);
   }
</script>

<Component on:close={handleCloseEvent} />

ts
type ComponentEvents<Component extends SvelteComponent> =
	Component extends SvelteComponent<any, infer Events>
		? Events
		: never;

ComponentPropspermalink

Convenience type to get the props the given component expects. Example:

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

	const props: ComponentProps<Component> = { foo: 'bar' }; // Errors if these aren't the correct props
</script>

ts
type ComponentProps<Component extends SvelteComponent> =
	Component extends SvelteComponent<infer Props>
		? Props
		: never;

ComponentTypepermalink

Convenience type to get the type of a Svelte component. Useful for example in combination with dynamic components using <svelte:component>.

Example:

<script lang="ts">
	import type { ComponentType, SvelteComponent } from 'svelte';
	import Component1 from './Component1.svelte';
	import Component2 from './Component2.svelte';

	const component: ComponentType = someLogic() ? Component1 : Component2;
	const componentOfCertainSubType: ComponentType<SvelteComponent<{ needsThisProp: string }>> = someLogic() ? Component1 : Component2;
</script>

<svelte:component this={component} />
<svelte:component this={componentOfCertainSubType} needsThisProp="hello" />

ts
type ComponentType<
	Component extends SvelteComponent = SvelteComponent
> = (new (
	options: ComponentConstructorOptions<
		Component extends SvelteComponent<infer Props>
			? Props
			: Record<string, any>
	>
) => Component) & {
	/** The custom element version of the component. Only present if compiled with the `customElement` compiler option */
	element?: typeof HTMLElement;
};

SvelteComponentpermalink

Base class for Svelte components with some minor dev-enhancements. Used when dev=true.

Can be used to create strongly typed Svelte components.

Example:permalink

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:

ts
import { SvelteComponent } from "svelte";
export class MyComponent extends SvelteComponent<{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:

<script lang="ts">
	import { MyComponent } from "component-library";
</script>
<MyComponent foo={'bar'} />

ts
class SvelteComponent<
	Props extends Record<string, any> = any,
	Events extends Record<string, any> = any,
	Slots extends Record<string, any> = any
> {…}

ts
[prop: string]: any;

ts
constructor(options: ComponentConstructorOptions<Props>);

ts
$destroy(): void;

ts
$capture_state(): void;

ts
$inject_state(): void;

SvelteComponentTypedpermalink

Use SvelteComponent instead. See PR for more information: https://github.com/sveltejs/svelte/pull/8512

ts
class SvelteComponentTyped<
	Props extends Record<string, any> = any,
	Events extends Record<string, any> = any,
	Slots extends Record<string, any> = any
> extends SvelteComponent<Props, Events, Slots> {}

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