chore: watch for messages changes in dev generate script (#15950)

* chore: watch for messages changes in dev generate script

* no need for this to be async

* tweak

* guard

* only create one timeout

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

packages/svelte/package.json

@@ -136,7 +136,7 @@
   ],
   "scripts": {
     "build": "node scripts/process-messages && rollup -c && pnpm generate:types && node scripts/check-treeshakeability.js",
-    "dev": "node scripts/process-messages && rollup -cw",
+    "dev": "node scripts/process-messages -w & rollup -cw",
     "check": "tsc --project tsconfig.runtime.json && tsc && cd ./tests/types && tsc",
     "check:watch": "tsc --watch",
     "generate:version": "node ./scripts/generate-version.js",

packages/svelte/scripts/process-messages/index.js

@@ -1,14 +1,19 @@
 // @ts-check
+import process from 'node:process';
 import fs from 'node:fs';
 import * as acorn from 'acorn';
 import { walk } from 'zimmerframe';
 import * as esrap from 'esrap';
 
+const DIR = '../../documentation/docs/98-reference/.generated';
+
+const watch = process.argv.includes('-w');
+
+function run() {
 	/** @type {Record<string, Record<string, { messages: string[], details: string | null }>>} */
 	const messages = {};
 	const seen = new Set();
 
-const DIR = '../../documentation/docs/98-reference/.generated';
 	fs.rmSync(DIR, { force: true, recursive: true });
 	fs.mkdirSync(DIR);
 
@@ -54,6 +59,7 @@ for (const category of fs.readdirSync('messages')) {
 			}
 
 			sorted.sort((a, b) => (a.code < b.code ? -1 : 1));
+
 			fs.writeFileSync(
 				`messages/${category}/${file}`,
 				sorted.map((x) => x._.trim()).join('\n\n') + '\n'
@@ -65,7 +71,10 @@ for (const category of fs.readdirSync('messages')) {
 			'<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->\n\n' +
 				Object.entries(messages[category])
 					.map(([code, { messages, details }]) => {
-					const chunks = [`### ${code}`, ...messages.map((message) => '```\n' + message + '\n```')];
+						const chunks = [
+							`### ${code}`,
+							...messages.map((message) => '```\n' + message + '\n```')
+						];
 
 						if (details) {
 							chunks.push(details);
@@ -407,3 +416,26 @@ transform('client-errors', 'src/internal/client/errors.js');
 	transform('server-errors', 'src/internal/server/errors.js');
 	transform('shared-errors', 'src/internal/shared/errors.js');
 	transform('shared-warnings', 'src/internal/shared/warnings.js');
+}
+
+if (watch) {
+	let running = false;
+	let timeout;
+
+	fs.watch('messages', { recursive: true }, (type, file) => {
+		if (running) {
+			timeout ??= setTimeout(() => {
+				running = false;
+				timeout = null;
+			});
+		} else {
+			running = true;
+
+			// eslint-disable-next-line no-console
+			console.log('Regenerating messages...');
+			run();
+		}
+	});
+}
+
+run();