chore: markdown rendering fixes (#1094)

* chore: markdown fixes

- handle `@param` on properties
- properly run markdown on `.js` files. Previously `.js` was treated as TS, which resulted in errors because the jsdoc was ignored. This was done this way because twoslash does not run on `js` by default, but there's a different way to make it still run on them while respecting jsdoc.

* set relevant compiler options

* see if that helps

* another one

Author: dummdidumm

apps/svelte.dev/content/docs/kit/30-advanced/25-errors.md

@@ -58,8 +58,7 @@ This throws an exception that SvelteKit catches, causing it to set the response
 You can add extra properties to the error object if needed...
 
 ```js
-import { error } from '@sveltejs/kit';
-
+// @filename: ambient.d.ts
 declare global {
 	namespace App {
 		interface Error {
@@ -68,7 +67,10 @@ declare global {
 		}
 	}
 }
+export {}
 
+// @filename: index.js
+import { error } from '@sveltejs/kit';
 // ---cut---
 error(404, {
 	message: 'Not found',

apps/svelte.dev/content/docs/svelte/02-runes/02-$state.md

@@ -45,12 +45,7 @@ todos[0].done = !todos[0].done;
 If you push a new object to the array, it will also be proxified:
 
 ```js
-// @filename: ambient.d.ts
-declare global {
-	const todos: Array<{ done: boolean, text: string }>
-}
-
-// @filename: index.js
+let todos = [{ done: false, text: 'add more todos' }];
 // ---cut---
 todos.push({
 	done: false,

packages/site-kit/src/lib/markdown/renderer.ts

@@ -498,6 +498,31 @@ async function convert_to_ts(js_code: string, indent = '', offset = '') {
 					if (code.original[end - 1] === ';') end -= 1;
 					code.appendLeft(end, ` satisfies ${satisfies}`);
 				}
+			} else if (
+				(ts.isPropertyAssignment(node) && ts.isArrowFunction(node.initializer)) ||
+				ts.isMethodDeclaration(node)
+			) {
+				if (type) {
+					throw new Error('@type on property methods does nothing');
+				}
+
+				const parameters = ts.isMethodDeclaration(node)
+					? node.parameters
+					: (node.initializer as ts.ArrowFunction).parameters;
+				for (let i = 0; i < parameters.length; i += 1) {
+					if (params[i] !== undefined) {
+						code.appendLeft(parameters[i].getEnd(), `: ${params[i]}`);
+					}
+				}
+
+				if (returns) {
+					const body = ts.isMethodDeclaration(node)
+						? node.body
+						: (node.initializer as ts.ArrowFunction).body;
+					let start = body!.getStart();
+					while (code.original[start - 1] !== ')') start -= 1;
+					code.appendLeft(start, `: ${returns}`);
+				}
 			} else {
 				throw new Error('Unhandled @type JsDoc->TS conversion: ' + js_code);
 			}
@@ -699,16 +724,20 @@ async function syntax_highlight({
 
 		try {
 			html = await codeToHtml(prelude + redacted, {
-				lang: 'ts',
+				lang: language,
 				theme,
 				transformers: check
 					? [
 							transformerTwoslash({
 								twoslashOptions: {
 									compilerOptions: {
+										allowJs: true,
+										checkJs: true,
 										types: ['svelte', '@sveltejs/kit']
 									}
-								}
+								},
+								// by default, twoslash does not run on .js files, change that through this option
+								filter: () => true
 							})
 						]
 					: []