Make Babel an optional peer dependency

.changeset/many-hotels-make.md

@@ -0,0 +1,7 @@
+---
+'@prefresh/vite': major
+---
+
+Make Babel an optional peer dependency
+
+`@babel/core` and `@prefresh/babel-plugin` are no longer hard dependencies of `@prefresh/vite`. Projects using modern versions of Vite no longer require Babel in their dependency tree.

packages/vite/package.json

@@ -36,22 +36,32 @@
   },
   "homepage": "https://github.com/preactjs/prefresh#readme",
   "dependencies": {
-    "@babel/core": "^7.22.1",
     "@prefresh/core": "^1.5.0",
-    "@prefresh/babel-plugin": "^0.5.2",
     "@prefresh/rolldown": "workspace:*",
     "@prefresh/utils": "^1.2.0",
     "@rollup/pluginutils": "^4.2.1",
     "rolldown": "^1.0.0-rc.12"
   },
   "devDependencies": {
+    "@babel/core": "^7.22.1",
+    "@prefresh/babel-plugin": "^0.5.2",
     "preact": "^10.26.10",
     "vite": "^8.0.0"
   },
   "peerDependencies": {
+    "@babel/core": "^7.22.1",
+    "@prefresh/babel-plugin": "^0.5.2",
     "preact": "^10.4.0 || ^11.0.0-0",
     "vite": ">=2.0.0"
   },
+  "peerDependenciesMeta": {
+    "@babel/core": {
+      "optional": true
+    },
+    "@prefresh/babel-plugin": {
+      "optional": true
+    }
+  },
   "publishConfig": {
     "provenance": true
   }

packages/vite/src/index.js

@@ -2,18 +2,38 @@ const { createFilter } = require('@rollup/pluginutils');
 
 const SCRIPT_LANG_RE = /\.(c|m)?(t|j)sx?$/;
 let babel;
+let babelLoadAttempted = false;
 let prefreshRolldownPromise;
 let viteSupportsHookFilters;
 let viteVersionParts;
 
-function loadBabel() {
-  babel ||= {
+function tryLoadBabel() {
+  if (babelLoadAttempted) return babel;
+  babelLoadAttempted = true;
+  try {
+    require.resolve('@babel/core');
+    require.resolve('@prefresh/babel-plugin');
+  } catch (err) {
+    if (err && err.code === 'MODULE_NOT_FOUND') return babel;
+    throw err;
+  }
+  babel = {
     transformSync: require('@babel/core').transformSync,
     prefreshBabelPlugin: require('@prefresh/babel-plugin'),
   };
   return babel;
 }
 
+function loadBabel() {
+  const loaded = tryLoadBabel();
+  if (loaded) return loaded;
+  throw new Error(
+    '@prefresh/vite needs @babel/core and @prefresh/babel-plugin to use the Babel transform path ' +
+      '(triggered by `parserPlugins` or by Vite < 8 without rolldown support). ' +
+      'Install them as dev dependencies, or upgrade to a Vite version with rolldown so the Oxc path is used instead.'
+  );
+}
+
 function loadPrefreshRolldown() {
   prefreshRolldownPromise ||= import('@prefresh/rolldown').then(
     ({ default: prefreshRolldown }) => prefreshRolldown
@@ -213,6 +233,10 @@ function prefreshBabelTransformPlugin(options = {}, forceBabel) {
         const hasSig = /\$RefreshSig\$\(/.test(code);
 
         if (hasReg || hasSig) return;
+
+        // Babel is an optional fallback on the Oxc path; if it's not
+        // installed we silently skip rather than failing the request.
+        if (!tryLoadBabel()) return;
       }
 
       const result = transform(code, id, parserPlugins);