Suggestion: TypeScript template should type check .ts files

[jbrunton]

I'm opening this issue for discussion, as I see there are different ways to implement this. I'd be happy to submit a PR if there's a consensus.

The problem

Currently, setupTypeScript.js adds a validate npm script to run svelte-check, and this catches type errors in .svelte files. However, it does not type check .ts files, and it wasn't until I noticed I had type errors on my main branch (despite CI running build and validate commands) that I realized this was the case. Admittedly, the docs for svelte-check make this pretty obvious in hindsight, but I had assumed that the default template for a TypeScript Svelte project would implement full type checking.

There are a couple of simple options which could be added to the setupTypeScript.js script which would fix this.

Option 1: Configure TypeScript plugin to fail on errors

The generated Rollup configuration does show warnings for type errors in .ts files, which hints at one solution to this: to add a noEmitOnError: true option to the Rollup TypeScript plugin config. However, with this approach, one ends up with type checking for different file types occurring during different phases of the build or CI: validate type checks .svelte files, and build type checks .ts files. I think this would also be confusing to new users.

Option 2: Add type checking to "validate" command

Perhaps the simplest fix is to instead modify the validate command like so:

  "validate": "svelte-check && tsc --noEmit"

Then, the same command type checks everything.

I can see that there are pros and cons to doing this: the scope of svelte-check is more than simply type checking, so it could be argued that this approach starts to conflate concerns. However, the naming of the validate script is fairly generic, which is why I assumed it would validate my entire project, and type checking is the minimum level of validation I would expect from this command for a TypeScript project. Modifying the validate command like this is also very obvious and easy for users to change if they wish to take a different approach.

Other questions

1. Is there a "recommended" solution to this problem from Svelte? (One of the above, or something I'm missing?)
2. Is there any reason not to amend the setup script to adopt one of these potential fixes? I'd be happy to submit a PR if it's helpful.

Thanks!

[dummdidumm]

The template was udpated some time ago, svelte-check now also checks TS/JS files.