---
title: How TypeScript checks a .tsrx file
description: TypeScript checks a TSX copy of each .tsrx file, and every error points back to the line you wrote.
sidebar: { label: Type checking }
---

Your editor underlines mistakes in `.tsrx` files now. But TypeScript has never heard of `.tsrx`. So who checks them?

A translation step does. It turns each `.tsrx` file into a TSX copy that TypeScript can read. TypeScript checks the copy, and each error points back to your line.

Press **Check**. Then pick `label="Home"` and check again.

<ToolTypecheckFigure />

Notice that TypeScript reads only the copy. Your `.tsrx` file stays as you wrote it.

## What gets checked

The copy keeps your real types. So a wrong prop is an error at the exact attribute.

```tsrx App.tsrx
import { Nav } from './Nav.tsrx';

export function App() @{
	<Nav label={42} />
}
```

`Nav` wants `label` to be a `string`. TypeScript reports that `number` is not assignable to `string`, and it underlines `label`.

The check covers the body of the file too. That includes attributes, event handlers, and element handles.

:::warning[Why is className an error?]
Markless uses the HTML name, `class`. The checker rejects `className` on HTML tags.
:::

## From the command line

Inside the Markless repo, `pnpm typecheck` checks the whole workspace. It wraps `tsc`, so `-p`, `--watch`, and every other `tsc` flag still work.

```bash
pnpm typecheck
```

A type error in a `.tsrx` body shows your file, line, and column. A file that does not parse fails the run too.

```text
counter.tsrx(9,8): error TS2322: Type 'string' is not assignable to type 'number'.
broken.tsrx(6,1): error TS91001: Markless TSRX parse error: Unexpected '}' in JSX text
Found 1 Markless TSRX compile error.
```

:::warning[Can I run this in my own app?]
Not yet. The published `@markless/typescript-plugin` ships the editor plugin only, with no command. In your app, the editor checks types. `npm run build` stops on Markless errors, but it does not run the type checker.
:::

**Next:** Types pass. But does the button still count when someone clicks it? [Testing →](/tooling/testing)