fix: key-value style messages broken after merging (#1717) (#1718)

* fix: key-value style messages broken after merging (#1717)

* test: missing test for pull request #1718 (issue #1717)

Author: chojnicki

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

@@ -2435,9 +2435,11 @@ export function createComposer(options: any = {}, VueI18nLegacy?: any): any {
       __legacy && __legacy.mergeLocaleMessage(locale, message)
     }
     const _message = { [locale]: message }
-    for (const key in _message) {
-      if (hasOwn(_message, key)) {
-        handleFlatJson(_message[key])
+    if (flatJson) {
+      for (const key in _message) {
+        if (hasOwn(_message, key)) {
+          handleFlatJson(_message[key])
+        }
       }
     }
     message = _message[locale]

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

@@ -1273,3 +1273,19 @@ test('issue #1615', async () => {
     `false (...but this should be false)`
   )
 })
+
+test('issue #1717', async () => {
+  const en = {
+    'a.b.c': 'Hello, Vue I18n'
+  }
+  const i18n = createI18n({
+    locale: 'en',
+    messages: {
+      en: {}
+    }
+  })
+  i18n.global.mergeLocaleMessage('en', en)
+  expect(i18n.global.getLocaleMessage('en')).toEqual({
+    'a.b.c': 'Hello, Vue I18n' // should not be transformed to nested object like in issue
+  })
+})