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TypeScript Support

Official blog post

Setup

1. Install the required packages and setting up your build

Starting fresh? Use the starter template which has a node script which sets it all up for you.

Adding it to an existing project? The official blog post explains how to do it.

2. Getting it to work in the editor

To tell us to treat your script tags as typescript, add a lang attribute to your script tags like so:

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

You may optionally want to add a svelte.config.js file - but it is not required as long as you only use TypeScript. Depending on your setup, this config file needs to be written either in ESM-style or CJS-Style.

ESM-style (for everything with "type": "module" in its package.json, like SvelteKit):

import sveltePreprocess from 'svelte-preprocess';

export default {
    preprocess: sveltePreprocess()
};

CJS-style:

const sveltePreprocess = require('svelte-preprocess');

module.exports = {
    preprocess: sveltePreprocess()
};

3. Restart the svelte language server

You will need to tell svelte-vscode to restart the svelte language server in order to pick up the new configuration.

Hit ctrl-shift-p or cmd-shift-p on mac, type svelte restart, and select Svelte: Restart Language Server. Any errors you were seeing should now go away and you're now all set up!

Typing components, authoring packages

When you provide a library, you also should provide type definitions alongside your code. You should not provide Svelte files that need preprocessors. So when you author a Svelte component library and write it in TypeScript, you should transpile the Svelte TS Code to JavaScript to provide JS/HTML/CSS-Svelte files. To type these components, place d.ts files next to their implementation. So for example when you have Foo.svelte, place Foo.svelte.d.ts next to it and tooling will aquire the types from the d.ts file. This is in line with how it works for regular TypeScript/JavaScript. Your Foo.svelte.d.ts should look something like this:

import { SvelteComponentTyped } from 'svelte';

export interface FooProps {
    propA: string;
    // ...
}

export interface FooEvents {
    click: MouseEvent;
    customEvent: CustomEvent<boolean>;
}

export interface FooSlots {
    default: { slotValue: string };
    named: { slotValue: string };
}

export default class Foo extends SvelteComponentTyped<FooProps, FooEvents, FooSlots> {}

SvelteKit's package command will give you these capabilities - transpiling and creating type definitions - out of the box: https://kit.svelte.dev/docs/packaging

Typing component events

When you are using TypeScript, you can type which events your component has in two ways:

The first and possibly most often used way is to type the createEventDispatcher invocation like this:

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

    const dispatch = createEventDispatcher<{
        /**
         * you can also add docs
         */
        checked: boolean; // Will translate to `CustomEvent<boolean>`
        hello: string;
    }>();

    // ...
</script>

This will make sure that if you use dispatch that you can only invoke it with the specified names and its types.

Note though that this will NOT make the events strict so that you get type errors when trying to listen to other events when using the component. Due to Svelte's dynamic events creation, component events could be fired not only from a dispatcher created directly in the component, but from a dispatcher which is created as part of another import. This is almost impossible to infer.

Troubleshooting / FAQ

I cannot use TS inside my script even when lang="ts" is present

Make sure to follow the setup instructions

How do I type reactive assignments? / I get an "implicitly has type 'any' error"

The following code may throw an error like Variable 'show' implicitly has type 'any' in some locations where its type cannot be determined., if you have stricter type settings:

<script lang="typescript">
    export let data: { someKey: string | null };

    $: show = !!data.someKey; // <-- `show` has type `any`
</script>

{#if show}hey{/if}

To type the variable, do this:

let show: boolean; // <--- added above the reactive assignment
$: show = !!data.someKey; // <-- `show` now has type `boolean`

How do I import interfaces into my Svelte components? I get errors after transpilation!

  • If you use svelte-preprocess BELOW v4.x and did NOT set transpileOnly: true, then make sure to have at least v3.9.3 installed, which fixes this.
  • If you don't use svelte-preprocess OR use transpileOnly: true (which makes transpilation faster) OR use v4.x, import interfaces like this: import type { SomeInterface } from './MyModule.ts'. You need a least TypeScript 3.8 for this.

Can I use TypeScript syntax inside the template/mustache tags?

At the moment, you cannot. Only script/style tags are preprocessed/transpiled. See this issue for more info.

Why is VSCode not finding absolute paths for type imports?

You may need to set baseUrl in tsconfig.json at the project root to include (restart the language server to see this take effect):

"compilerOptions": {
    "baseUrl": "."
  }
}

I'm using an attribute/event on a DOM element and it throws a type error

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: Create a additional-svelte-typings.d.ts file:

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.

You need svelte-check version 3 / VS Code extension version 106 for this. Also see the next section.

I'm getting deprecation warnings for svelte.JSX / I want to migrate to the new typings

Since svelte-check version 3 and VS Code extension version 106, a different transformation is used to get intellisense for Svelte files. This also leads to the old way of enhancing HTML typings being deprecated. You should migrate all usages of svelte.JSX away to either svelte/elements or the new svelteHTML namespace.

If you used svelte.JSX in your library to express that you have a component that wraps a HTML element, use svelte/elements (part of Svelte since version 3.55) instead:

import { SvelteComponentTyped } from 'svelte';
+import { HTMLButtonAttributes } from 'svelte/elements';

export MyFancyButton extends SvelteComponentTyped<
-  svelte.JSX.HTMLAttributes<HTMLElement>
+  HTMLButtonAttributes
> {}
<script lang="ts">
+  import { HTMLButtonAttributes } from 'svelte/elements';
-  interface $$Props extends svelte.JSX.HTMLAttributes<HTMLElement> {
+  interface $$Props extends HTMLButtonAttributes {
    foo: string;
  }

  export let foo: string;
</script>

<button {...$$props}>
  <slot />
</button>

If you used svelte.JSX in your project to enhance the HTML typings, use the svelteHTML namespace instead:

-declare namespace svelte.JSX {
+declare namespace svelteHTML {
    // enhance elements
    interface IntrinsicElements {
        'my-custom-element': { someattribute: string };
    }
    // enhance attributes
    interface HTMLAttributes<T> {
        // If you want to use on:beforeinstallprompt
-        onbeforeinstallprompt?: (event: any) => any;
+        '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
    }
}

I'm unable to use installed types (for example through @types/..)

You are most likely extending from Svelte's @tsconfig/svelte base config in your tsconfig.json, or you did set "types": [..] in your tsconfig. In both cases, a "types": [..] property is present. This makes TypeScript prevent all ambient types which are not listed in that types-array from getting picked up. The solution is to enhance/add a types section to your tsconfig.json:

{
    "compilerOptions": {
        // ..
        "types": ["svelte", "..<your installed type>.."]
    }
}

We are looking for ways to make the types definition in @tsconfig/svelte unnecessary, so you don't have those issues in the future.

I'm getting weird behavior when using "module": "CommonJS"

Don't set the module to CommonJS, it will result in wrong transpilation of TypeScript to JavaScript. Moreover, you shouldn't set this anyway as CommonJS is a module format for NodeJS which is not understood by the Browser. For more technical details, see this issue comment.