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Svelte(Kit) TypeScript Showcase + general TypeScript tipps

This repository shows how Svelte and SvelteKit work together with TypeScript.

This repository should offer an overview of all TypeScript related topics for Svelte and SvelteKit
Feel free to contribute by creating a PR or open an issue if you think something is missing.

Who am I

Hi, I'm Ivan, a passionate webdeveloper.

I recently have been working more intensively with TypeScript when I have created an internationalization library focusing on developer experience with strong typesafety features: typesafe-i18n

I know and love Svelte for a few years now. Over the years I saw how my development workflow improved and now together with TypeScript, I'm able to build real business applications with confidence. When I started with Svelte, the missing TypeScript support always bothered me. And once I could use TypeScript in my Svelte projects I still found it not so easy because of some missing documentation. That's why I decided to create this repository with some examples that should help you learn the concepts better. I hope you find it useful.

Become a sponsor ❤️ if you want to support my open source contributions.

Overview

Project Setup

In order to get the best development expeience you should use VS Code as your IDE and install the official Svelte extension. The extension offers you a variety of features like code-completions, hover infos, syntax error highlighting and much more.

I also recommend to install the Error Lens extension that displays error-messages inline next to the actual code.

Get Started

You can create a new SvelteKit project by following the Getting started guide in the official docs.

npm init svelte@next my-app
cd my-app
npm install
npm run dev

The npm init svelte@next my-app command starts an interactive project-setup process where you get asked a few questions. Of course you should select the TypeScript option.

I also recommend enabling the eslint and prettier options.

Configure TypeScript

In the root of the generated folder, you should see a tsconfig.json file. I recommend you to configure TypeScript as strict as possible to benefit from the advanced type-checking features.

As of writing these are the strictest options I know. This list will grow with new TypeScript releases.
Please create a PR if you know more options that should be enabled.

{
   "compilerOptions": {
      "strict": true,
      "allowUnreachableCode": false,
      "exactOptionalPropertyTypes": true,
      "noImplicitAny": true,
      "noImplicitOverride": true,
      "noImplicitReturns": true,
      "noImplicitThis": true,
      "noFallthroughCasesInSwitch": true,
      "noUncheckedIndexedAccess": true
   }
}

Not all options may fit your coding style. You can always remove some of them but be aware that this could eliminate some cool type-checking features.

How does TypeScript work inside Svelte-Components?

Per default the Svelte-compiler only understands plain HTML, CSS and JavaScript. But we can add support for other languages to the compiler via custom preprocessors. Luckily, we don't have to write our own preprocessor because there exists already an official package we can use. svelte-preprocess enables us to use TypeScript and also custom CSS syntax like SCSS or PostCSS without much effort. If you take a look at the svelte.config.js file, you see that this was arleady set up for us.

The next step will be to create our first Component and use TypeScript inside the script-tag.

<script>
   export let name: string
</script>

Hello {name}!

If everything is correctly set up, you should see an error message telling you something like 'Unexpected Token'. That's because we have to tell the preprocessor that we want to use TypeScript syntax. We can do that by adding the lang="ts" attribute to our script tag.

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

Hello {name}!

That's it. You are now ready to write TypeScript code inside your Svelte-components.

imports from .ts files

You can also import functions from other TypeScript files like you would in a normal .ts file. If you import types from another file, make sure you use the import type-syntax

<script>
   import { myFunction } from './my-file'
   import type { MyFunctionType } from './my-file'
</script>

if you are using a TypeScript version >= 4.5.x you can also write it like this:

<script>
   import { myFunction, type MyFunctionType } from './my-file'
</script>

type-checking the whole project

You should know that even if you have TypeScript errors in your code, the Svelte-compiler will generate your component (if the code contains valid TypeScript syntax) and the browser will run the code normally. That's because the preprocessor only transpiles TypeScript to JavaScript and doesn't perform any type-checking. That's a reason why we should use the Svelte extension that will perform the type-checking for the components we have opened in VS Code. Performing a global type-check for all components each time you save a file may be too resource intensive for most computers, so only errors for opened files will show up.

This approach has a downside: If we change something in a component and haven't opened the file where we use that specific component, we won't get notified about errors. Again luckily for us there exists a solution to this problem: a package called svelte-check.

If you take a look at the scripts section of the package.json, you will see that it is already configured for us. We simply can run the following command to perform a check of the whole project:

npm run check

You should include this svelte-check script in your CI/CD process to get notified if your components contain TypeScript errors.

path aliases

You may see imports from paths that are not npm modules and also not relative paths e.g. $components/Component.svelte. These are called path aliases and you can define them yourself if you like.

Let's say instead of using a relative file import like ../../../components/Component.svelte you want to use the alias import $components/Component.svelte. To do that, you only need to define the desired alias in your svelte.config.js file:

/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    alias: {
      $components: 'src/components',
    }
  }
};

These aliases are automatically passed to Vite and TypeScript. You can define as many aliases as you want.

Examples

The next sections contain a list of all examples included in this repo. It is recommended to explore the examples inside VS Code to have proper syntax-highlighting in place.

  • the title links directly to the folder, where the example is located
  • there is also a short description what is included in the example
  • a link to the official documentation to gain more information (if available)
  • most examples contain a Component.svelte and an Usage.svelte file to show it in action

I recommend going over the examples in this order since some examples build on top of each other.

Svelte

props

https://svelte.dev/docs#component-format-script-1-export-creates-a-component-prop

This chapter teaches you everything about how you can use TypeScript to improve your component's props.

dom

In this chapter you will learn how to interact directly with DOM-elements.

events

https://svelte.dev/docs#run-time-svelte-createeventdispatcher

This chapter shows how you can define events that a component emits.

slots

https://svelte.dev/docs#template-syntax-slot

external components

  • SvelteComponentTyped: how to write type definitions for external components

  • svelte-kit package: how to create a Svelte component library

    https://kit.svelte.dev/docs/packaging

    SvelteKit includes an easy way to export single components or component libraries. Just run npm run package and SvelteKit will export all your components from the src/lib directory, together with TypeScript definitions into the package folder. This folder then also contains a generated package.json file. After that, you only need to run npm publish inside this folder to upload the library to npm.

state management

SvelteKit

In this chapter you get to know how to type the backend of your SvelteKit application.

  • hooks: how to intercept and modify requests

    https://kit.svelte.dev/docs/hooks#server-hooks

    The src/hooks.server.ts file can export four functions. The type of these functions have the same name like the function and get exported from @sveltejs/kit.

    import type { HandleFetch, Handle, HandleServerError } from '@sveltejs/kit'
    
    export const handle: Handle = async ({ event, resolve }) => {
       /* implementation */
    }
    
    export const handleError: HandleServerError = async ({ error, event }) => {
       /* implementation */
    }
    
    export const handleFetch: HandleFetch = async (request) => {
       /* implementation */
    }

    The handle and getSession function will have access to the locals and the session object. To let TypeScript know how the type of these objects look like, you need to go into the src/app.d.ts file and update the already existing interfaces there.

    Since these types will be shared across multiple files and functions, it makes sense to define them just a single time. SvelteKit is configured in a way that it automatically uses those types for all functions.

  • endpoints: how to use SvelteKit as an API-endpoint

    https://kit.svelte.dev/docs/routing#endpoints

    We can use RequestHandler to type our endpoints. It expects a single generics:

    1. The type describes the shape the returned value will have.

    src/routes/product/[id].ts

    import type { RequestHandler } from './$types'
    import type { Product } from '$models/product.model'
    import db from '$db'
    
    type OutputType = { product: Product }
    
    export const GET: RequestHandler<OutputType> = async ({ params }) => {
       const data = await db.getProjectById(params.id)
    
       return {
          body: {
             product: data,
          },
       }
    }

    Note: SvelteKit auto-generates the ./$types folder for us. We can use it to get a RequestHandler type that already has the correct shape for the params object.

  • load function: how to load data before the page gets rendered

    https://kit.svelte.dev/docs/loading

    Use the Load inferface type load functions in your route. It expects two generics:

    1. The first type will be the output type of your endpoint if available.
      If no GET-endpoint is defined, the props object will be undefined.
    2. The second type describes the shape the returned value will have.

    src/routes/product/[id]/+page.svelte

    import type { PageLoad, PageLoadData } from './$types'
    import type { GET } from './[id]'
    
    type OutputProps = PageLoadData & { id: string }
    // the same as
    // type OutputProps = {
    //    id: string
    //    product: Product
    // }
    
    export const load: PageLoad<OutputProps> = async ({ params, props }) => {
       return {
          props: {
             id: params.id,
             product: props.product,
          },
       }
    }

    src/routes/product/[id]/+page.svelte

    <script lang="ts">
       import type { PageData } from './$types'
    
       // PageData = { id: string; product: Product }
       export let data: PageData
    </script>

    Note: SvelteKit auto-generates the ./$types folder for us. We can use it to get a Load type that already has the correct shape for the params object.

  • auto generated types
    SvelteKit creates some types automatically. Useful when you want to type your Endpoints and Load functions. Those types contain a typed params object depending on the route folder structure you use. The types are generated inside the ./$types folder.
    The types are generated when you run the dev server npm run dev. If you just want to generate the types, without running the dev server you can run npx svelte-kit sync. When you run npm install, the types will be generated automatically because the SvelteKit runs a post-install script that generates the files.

TypeScript tipps

Here are some examples how you could improve your code base by adding stronger type definitions for common use cases. Not everything needs to be typed that strong. Stricter type definitions will also add complexity you need to maintain, but it certainly improves the devloper experience when using strong typed functions within the code base.

#1 union types

Example

You can use union types to narrow down types in the control flow of your application. Somewhere you probably need to fetch data from an api. The fetch function will probably either return data or an error. It is not wrong to model it like in the following example:

example
interface ApiResponse<T> {
   success: boolean
   data: T | undefined
   error: Error | undefined
}

But it doesn't work that well when you now want to access the data object because its type definition also contains undefined:

let response: ApiResponse<string>

if (response.success) {
   // `response.data` is of type `string | undefined`
} else {
   // `response.error` is of type `Error | undefined`
}
improved example

We can improve the example by spitting our interface and then using an union type:

// will contain data but no Error
export interface SuccessResponse<T> {
   success: true
   data: T
   error: undefined
}

// will contain an Error but no data
export interface ErrorResponse {
   success: false
   data: undefined
   error: Error
}

// our union type
export type ApiResponse<T> = SuccessResponse<T> | ErrorResponse

If we now access the data we will see that its type is no longer undefined:

let response: ApiResponse<string>

if (response.success) {
   // `response.data` is of type `string`
} else {
   // `response.error` is of type `Error`
}

So whenever you know that something is either A or either B you should also model it that way by splitting the model into two different interfaces and then use an union type to.

You can learn more about union types in the official TypeScript documentation

#2 extend existing type definitions

Example
a more complex example you don't need to fully understand. But this example can show you what is possible with TypeScript if you know how to use it ;)

Sometimes it is possible that a library contains missing or incomplete type definitions. You could either use // @ts-ignore comments and live with it or you can write the type declaration yourself.

Create a *.d.ts file somewhere in your src folder and use the following syntax:

import 'package' // `'package'` is the library we want to extend

declare module 'package' {
   // we re-declare the module
   // we add the missing function or override the existing one
   export declare function someFunction(): boolean
}

You can now use it inside your code:

import { someFunction } from 'package'

const result = someFunction()
// result: boolean

#3 wrap functions that have no strong typings

Example

Whenever you are using a function that has no or not so good TypeScript definitions or whenever you need to cast something everytime you use a function, you should wrap it into a new function and add type definitions there.

example
import { getContext } from 'svelte'

// per default the return type is `unknown`
const c1 = getContext('my context')

// we need to pass a generic to let `TypeScript` know what we expect
const c2 = getContext<string>('my context')

// oops typo!
const c3 = getContext<string>('my comtext')

The usage of getContext in the example above has three issues:

  1. you need to specify the return type whenever you call the function, so you would need to check where the context gets set and copy the type definition from there
  2. when you refactor the context to hold different data, you need to update the type definition everywhere
  3. you could easily introduce a typo because the parameter of the function is typed as a generic string
improved example

We can eliminate the issues mentioned above by wrapping getContext into a new function

import { getContext } from 'svelte'

const getMyContext = () => getContext<string>('my context')

const c = getMyContext() // typed as `string`

We now have a single function that is responsible for the type (1) and the context name (3). And we use that function when we want to access the data. When refactoring (2) we only need to change it in a single place (and let TypeScript tell you if it's now getting used in a wrong way).

#4 use opaque-types for unique types

Example

Some types may look similar to another type but they are not actually related. If you are working with databases the ID field would be such a case.

example

Typing the id of your DB model as a string is probably not wrong because from a technical perspective they are strings. But it is also a string for other models of your DB.

interface Product {
   id: string
   author: string
}

interface Category {
   id: string
   name: string
}

const book: Product = { id: '1', /* ...rest */ }
const category: Category = { id: '1', /* ...rest */ }

const findById(productId): Product | undefined = { /* implementation */ }

findById(product.id) // valid

findById(category.id)  // we query the product DB with an id from the category DB
// this will probably always return `undefined` if your DB uses random IDs

You could introduce potential bugs if you are not careful. In the case above it probably is clear because we have named the data variables clearly, but what if you name the variable just result and then use the findById function. At first glance it looks good, but it is actually wrong.

improved example

We can improve this by giving each model its unique ID type.

// in the next two lines we define an `opaque type` for our ProductId
declare const _productId: unique symbol
export type ProductId = string & { readonly [_productId]: never }
// we define our type as `string` but with additional meta-information
// that tell TypeScript that this is a unique string

// this type will behave like a `string`, so you can use it in functions that
// expect a `string`, but it doesn't work the other way around. You can't pass
// a normal `string` to a function that expects a certain `opaque type`

// of course you can use `opaque types` also for `numbers` and other types


interface Product {
   id: ProductId
   author: string
}

// we also define a CategoryId as an `opaque type`
declare const _categoryId: unique symbol
type CategoryId = string & { readonly [_categoryId]: never }

interface Category {
   id: CategoryId
   name: string
}

// in this case we need to cast it, because we are hardcoding the IDs
// in a real world scenario the data gets loaded on runtime
// from the DB and no casting is needed there
const book: Product = { id: '1' as ProductId, /* ...rest */ }
const category: Category = { id: '1' as CategoryId, /* ...rest */ }

const findById(id: ProductId): Product | undefined = { /* implementation */ }

findById(book.id) // valid

findById(category.id) // TypeScript shows an error:
// Argument of type 'CategoryId' is not assignable to parameter of type 'ProductId'

Now TypeScript is able to tell you that something is wrong when you pass in the wrong ID.

This is not only useful for IDs but also for other string types that e.g. have a special meaning. Such examples could be:

  • LocalizedString to make sure a Button component only accepts internationalized strings
  • SanitizedHtmlString so you know it contains no potential unsafe characters
  • ValidatedString that tells you it's content was checked and marked as valid

#5 use tagged template literals to narrow down your string types

Like you already have seen in the example from TS-tipp #4, strings are really generic and can hold any kind of data. Luckily TypeScript is flexible enough to let us define which shape we expect the data.

Example

// a list of possible options (`enum`-like)
type Options = 'A' | 'B' | 'C'

// `numbers` wrapped into a `string`
type NumberString = `${number}`

// really simple check if the format looks like an email
type EmailString = `${string}@${string}.${string}`

// simple check if it is an url
type LinkString = `http${'s' | ''}://${string}`

// simple check if it could be in the format of an IPv4 address
// note: we only can tell we expect any `number` but we cannot define value ranges yet
type IpStringSimple = `${number}.${number}.${number}.${number}`

// useful if you are handling multiple date formats from different apis
type Api1DateTimeString = `${number}-${number}-${number} ${number}:${number}:${number}`
// e.g. '2021-03-01 13:00:00'

type Api2Timestamp = `${number}` // e.g. '1635494400000'

// ... and many more potential use-cases

If we assign a value that doesn't match the definition of the shape TypeScript will throw an error.

Conclusion

You should now have a feeling what is possible with TypeScript and how you can use it within your Svelte and SvelteKit applications. You probably have learned something new about TypeScript as well. Just because the examples are listed here doesn't mean you need to type everything as strict as possible. For some cases it makes sense to be stricter and sometimes beeing so strict will introduce complexity you need to maintain over the lifetime of a project.

JSDoc comments

You probably don't need TypeScript directly to profit from a strong type-checking experience inside your Svelte and SvelteKit applications. VS Code and the Svelte extension can also offer help if you annotate your components with JSDoc comments.

Here is a simple example:

  • JSDoc:

    <script>
       /** @type {string} */
       export let name
    </script>
    
    Hello {name}!
  • TypeScript:

    <script lang="ts">
       export let name: string
    </script>
    
    Hello {name}!

It is up to you which syntax you prefer. Some parts of the SvelteKit codebase are written in plain JavaScript files annotated with JSDoc comments.

I would suggest directly using ´TypeScript´ because if you need more complex types, you will need to write declaration files in TypeScript syntax. It will be harder for you to write them if you are not used to the TypeScript syntax.

Also if you need to use generics inside your JSDoc comments, you may find the syntax a bit messy:

  • JSDoc:
/**
 * @typedef { import('./my-types').Type1 } Type1,
 * @typedef { import('./my-types').GenericType<Type1> } Type1GenericType,
 */

/**
 * @param {Type1GenericType} param
 * @returns {void}
 */
export const myFunction = (param) => {
   // ... implementation
}
  • TypeScript:
import type { Type1, GenericType } from ('./my-types')

export const myFunction = (param: GenericType<Type1>): void => {
   // ... implementation
}

See the official documentation to learn more about the JSDoc-syntax.

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This repository shows how Svelte and SvelteKit work together with TypeScript.

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