Why you should use TypeScript in your next SvelteKit projects

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Justin Ahinon

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SvelteKit version: 1.11.0

Svelte version: 3.57.0

I remember a few years back when TypeScript started being a thing, I was a bit skeptical about it.

For me, it was yet another thing to learn on top of JavaScript. I thought it'd just be a trend that will eventually die out. Fast forward to today, I'm a big fan of TypeScript. I use it in most of my JavaScript projects, and I heavily advocate for it when I pick up existing projects.

In my opinion, one of the things that makes it easy to adopt TypeScript on new projects is the integration with tools that we used day to day . You might be wondering: how easy it is to integrate TypeScript with my current stack?

Well, if you're using SvelteKit, it's pretty easy. In this post, I'm trying to convince you to use TypeScript in your next SvelteKit projects .

It’s easy to setup TypeScript in new SvelteKit projects

Setting up TypeScript in a new SvelteKit project is just a matter of a command line choice. When you're creating a new project with  create-svelte , you're prompted to choose between JavaScript and TypeScript.

Choosing TypeScript will create a new project with TypeScript already set up (this includes the configuration, types definitions for Svelte, SvelteKit, your own components, load functions, etc).

Choosing TypeScript syntax with create-svelte

It’s easy to use

You can use TypeScript in two categories of places in your SvelteKit project:

  • .svelte  files (components)

  • .ts  files (load functions, actions, API routes, stores, utils, etc)

For the second option, it's pretty obvious, you just need to create a  .ts  file and start writing your code. For the first option, you can use TypeScript in your components by using the  <script lang="ts">  tag. This is the same as using the  <script>  tag, but with the added benefit of type safety.

Here is what a Svelte component with TypeScript looks like:


You get real E2E type safety across your application

What I really like about the way SvelteKit handles TypeScript is how easily it makes types flow across your application . For instance, you define a load function that returns a list of players:


Now, whenever you use the  players  property in your component, you get type safety without needing to take any extra steps:


The each block above will trigger a type error because we are trying to access the  names  property of the  player  variable, which doesn't exist. This is because the  Player  interface only has a  name  property, not a  names  property.

Now, you can even use TypeScript without thinking about it

For a few days now, if you are using VSCode, you don't even need to import type definitions for page data, actions, etc. in your Svelte components. The same goes for the types of load functions, actions, etc. in your  .ts  files.

The Svelte team has used some smart tricks combining the Svelte language server, the VSCode extension, and the TypeScript plugin used in Svelte to infer 'automagically' the types in your components and TypeScript files. So, in the above example, you don't need to import the  PageData  type in your component in order to get type safety.

You can just have:


The type of  players  will be inferred automatically.

Even better, in the  /src/routes/players/+page.server.ts  file, you don't need to import the  PageServerLoad  for the function to be typed correctly. You can just have:


The  load  function takes some parameters like  request  and  params  . Before, you needed to import its type to get type safety, intelliSense, and auto completions for these parameters. Now, you don't need to do that anymore. Just add the parameters to the function, and you're good to go. TypeScript will infer the types for you, yells at you if you're doing something wrong, and gives you auto-completions.

Read more about this feature here: https://svelte.dev/blog/zero-config-type-safety

It's cool, huh?

On top of all these technical benefits, using TypeScript is just cool, and using it with SvelteKit is even cooler. It really provides some joy working with it. And I think it's worth at least trying it out.

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