improvement: tweak vue-i18n for SSR (#929)

Author: kazupon

packages/vue-i18n-core/src/composer.ts

@@ -10,7 +10,8 @@ import {
   isBoolean,
   isPlainObject,
   isObject,
-  assign
+  assign,
+  inBrowser
 } from '@intlify/shared'
 import {
   isTranslateFallbackWarn,
@@ -2346,7 +2347,7 @@ export function createComposer(options: any = {}, VueI18nLegacy?: any): any {
   composerID++
 
   // watch root locale & fallbackLocale
-  if (__root) {
+  if (__root && inBrowser) {
     watch(__root.locale, (val: Locale) => {
       if (_inheritLocale) {
         _locale.value = val

packages/vue-i18n-core/src/i18n.ts

@@ -8,6 +8,7 @@ import {
   isRef
 } from 'vue'
 import {
+  inBrowser,
   isEmptyObject,
   isBoolean,
   warn,
@@ -929,9 +930,13 @@ function setupLifeCycle(
     }
     vm._i18n = _i18n
     vm._i18n_bridge = true
-    vm._i18nWatcher = vm._i18n.watchI18nData()
-    if (vm._i18n._sync) {
-      vm._localeWatcher = vm._i18n.watchLocale()
+
+    // browser only
+    if (inBrowser) {
+      vm._i18nWatcher = vm._i18n.watchI18nData()
+      if (vm._i18n._sync) {
+        vm._localeWatcher = vm._i18n.watchLocale()
+      }
     }
 
     let subscribing = false

packages/vue-i18n-core/test/composer.test.ts

@@ -1,3 +1,6 @@
+/**
+ * @jest-environment jsdom
+ */
 /* eslint-disable @typescript-eslint/no-empty-function, @typescript-eslint/no-explicit-any */
 
 // utils