fix: allow to pass in TS preference to migration (#13929)

First half of https://github.com/sveltejs/kit/issues/12880 - the idea is for the migration script to check for a tsconfig.json and then set it to `true` if one is found

.changeset/giant-wombats-allow.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow to pass in TS preference to migration

packages/svelte/src/compiler/migrate/index.js

@@ -39,10 +39,10 @@ class MigrationError extends Error {
  * May throw an error if the code is too complex to migrate automatically.
  *
  * @param {string} source
- * @param {{filename?: string}} [options]
+ * @param {{ filename?: string, use_ts?: boolean }} [options]
  * @returns {{ code: string; }}
  */
-export function migrate(source, { filename } = {}) {
+export function migrate(source, { filename, use_ts } = {}) {
 	let og_source = source;
 	try {
 		has_migration_task = false;
@@ -115,9 +115,12 @@ export function migrate(source, { filename } = {}) {
 			derived_components: new Map(),
 			derived_labeled_statements: new Set(),
 			has_svelte_self: false,
-			uses_ts: !!parsed.instance?.attributes.some(
-				(attr) => attr.name === 'lang' && /** @type {any} */ (attr).value[0].data === 'ts'
-			)
+			uses_ts:
+				// Some people could use jsdoc but have a tsconfig.json, so double-check file for jsdoc indicators
+				(use_ts && !source.includes('@type {')) ||
+				!!parsed.instance?.attributes.some(
+					(attr) => attr.name === 'lang' && /** @type {any} */ (attr).value[0].data === 'ts'
+				)
 		};
 
 		if (parsed.module) {

packages/svelte/tests/migrate/samples/slot-use_ts/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	use_ts: true
+});

packages/svelte/tests/migrate/samples/slot-use_ts/input.svelte

@@ -0,0 +1 @@
+<slot />

packages/svelte/tests/migrate/samples/slot-use_ts/output.svelte

@@ -0,0 +1,9 @@
+<script>
+	interface Props {
+		children?: import('svelte').Snippet;
+	}
+
+	let { children }: Props = $props();
+</script>
+
+{@render children?.()}

packages/svelte/tests/migrate/test.ts

@@ -6,6 +6,7 @@ import { suite, type BaseTest } from '../suite.js';
 
 interface ParserTest extends BaseTest {
 	skip_filename?: boolean;
+	use_ts?: boolean;
 	logs?: any[];
 	errors?: any[];
 }
@@ -32,7 +33,8 @@ const { test, run } = suite<ParserTest>(async (config, cwd) => {
 	}
 
 	const actual = migrate(input, {
-		filename: config.skip_filename ? undefined : `output.svelte`
+		filename: config.skip_filename ? undefined : `output.svelte`,
+		use_ts: config.use_ts
 	}).code;
 
 	if (config.logs) {

packages/svelte/types/index.d.ts

@@ -1290,8 +1290,9 @@ declare module 'svelte/compiler' {
 	 * May throw an error if the code is too complex to migrate automatically.
 	 *
 	 * */
-	export function migrate(source: string, { filename }?: {
+	export function migrate(source: string, { filename, use_ts }?: {
 		filename?: string;
+		use_ts?: boolean;
 	} | undefined): {
 		code: string;
 	};