fix: allow empty default message (#1849)

Author: kazupon

packages/core-base/src/translate.ts

@@ -649,8 +649,11 @@ export function translate<
         : (!messageCompiler ? () => key : key)
       : fallbackFormat // default by `fallbackFormat` option
         ? (!messageCompiler ? () => key : key)
-        : ''
-  const enableDefaultMsg = fallbackFormat || defaultMsgOrKey !== ''
+        : null
+  const enableDefaultMsg =
+    fallbackFormat ||
+    (defaultMsgOrKey != null &&
+      (isString(defaultMsgOrKey) || isFunction(defaultMsgOrKey)))
   const locale = getLocale(context, options)
 
   // escape params

packages/core-base/test/translate.test.ts

@@ -120,6 +120,7 @@ describe('default option', () => {
     expect(translate(ctx, 'hello', 'hello, default message!')).toEqual(
       'hello, default message!'
     )
+    expect(translate(ctx, 'hello', '')).toEqual('')
   })
 
   test('boolean true', () => {

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

@@ -734,6 +734,16 @@ describe('t', () => {
     await nextTick()
     expect(t('hello')).toEqual(JA_HELLO)
   })
+
+  test('default msg', () => {
+    const { t } = createComposer({
+      locale: 'en',
+      messages: {
+        en: { hello: 'Hello!' }
+      }
+    })
+    expect(t('foo', '')).toEqual('')
+  })
 })
 
 describe('rt', () => {