fix(localize): support fallback locale

packages/localize/src/LocalizeManager.js

@@ -16,6 +16,7 @@ export class LocalizeManager extends LionSingleton {
     }
 
     this._autoLoadOnLocaleChange = !!params.autoLoadOnLocaleChange;
+    this._fallbackLocale = params.fallbackLocale;
     this.__storage = {};
     this.__namespacePatternsMap = new Map();
     this.__namespaceLoadersCache = {};
@@ -163,13 +164,24 @@ export class LocalizeManager extends LionSingleton {
     return loader;
   }
 
-  _getNamespaceLoaderPromise(loader, locale, namespace) {
+  _getNamespaceLoaderPromise(loader, locale, namespace, fallbackLocale = this._fallbackLocale) {
     return loader(locale, namespace).catch(() => {
       const lang = this._getLangFromLocale(locale);
       return loader(lang, namespace).catch(() => {
+        if (fallbackLocale) {
+          return this._getNamespaceLoaderPromise(loader, fallbackLocale, namespace, false).catch(
+            () => {
+              const fallbackLang = this._getLangFromLocale(fallbackLocale);
+              throw new Error(
+                `Data for namespace "${namespace}" and current locale "${locale}" or fallback locale "${fallbackLocale}" could not be loaded. ` +
+                  `Make sure you have data either for locale "${locale}" (and/or generic language "${lang}") or for fallback "${fallbackLocale}" (and/or "${fallbackLang}").`,
+              );
+            },
+          );
+        }
         throw new Error(
           `Data for namespace "${namespace}" and locale "${locale}" could not be loaded. ` +
-            `Make sure you have data for locale "${locale}" and/or generic language "${lang}".`,
+            `Make sure you have data for locale "${locale}" (and/or generic language "${lang}").`,
         );
       });
     });

packages/localize/test/LocalizeManager.test.js

@@ -213,7 +213,7 @@ describe('LocalizeManager', () => {
       });
     });
 
-    it('fallbacks to language file if locale file is not found', async () => {
+    it('loads generic language file if locale file is not found', async () => {
       setupFakeImport('./my-component/en.js', { default: { greeting: 'Hello!' } });
 
       manager = new LocalizeManager();
@@ -240,13 +240,69 @@ describe('LocalizeManager', () => {
         expect(e).to.be.instanceof(Error);
         expect(e.message).to.equal(
           'Data for namespace "my-component" and locale "en-GB" could not be loaded. ' +
-            'Make sure you have data for locale "en-GB" and/or generic language "en".',
+            'Make sure you have data for locale "en-GB" (and/or generic language "en").',
         );
         return;
       }
 
       throw new Error('did not throw');
     });
+
+    describe('fallback locale', () => {
+      it('can load a fallback locale if current one can not be loaded', async () => {
+        manager = new LocalizeManager({ fallbackLocale: 'en-GB' });
+        manager.locale = 'nl-NL';
+
+        setupFakeImport('./my-component/en-GB.js', { default: { greeting: 'Hello!' } });
+
+        await manager.loadNamespace({
+          'my-component': locale => fakeImport(`./my-component/${locale}.js`),
+        });
+
+        expect(manager.__storage).to.deep.equal({
+          'nl-NL': {
+            'my-component': { greeting: 'Hello!' },
+          },
+        });
+      });
+
+      it('can load fallback generic language file if fallback locale file is not found', async () => {
+        manager = new LocalizeManager({ fallbackLocale: 'en-GB' });
+        manager.locale = 'nl-NL';
+
+        setupFakeImport('./my-component/en.js', { default: { greeting: 'Hello!' } });
+
+        await manager.loadNamespace({
+          'my-component': locale => fakeImport(`./my-component/${locale}.js`),
+        });
+
+        expect(manager.__storage).to.deep.equal({
+          'nl-NL': {
+            'my-component': { greeting: 'Hello!' },
+          },
+        });
+      });
+
+      it('throws if neither current locale nor fallback locale are found', async () => {
+        manager = new LocalizeManager({ fallbackLocale: 'en-GB' });
+        manager.locale = 'nl-NL';
+
+        try {
+          await manager.loadNamespace({
+            'my-component': locale => fakeImport(`./my-component/${locale}.js`),
+          });
+        } catch (e) {
+          expect(e).to.be.instanceof(Error);
+          expect(e.message).to.equal(
+            'Data for namespace "my-component" and current locale "nl-NL" or fallback locale "en-GB" could not be loaded. ' +
+              'Make sure you have data either for locale "nl-NL" (and/or generic language "nl") or for fallback "en-GB" (and/or "en").',
+          );
+          return;
+        }
+
+        throw new Error('did not throw');
+      });
+    });
   });
 
   describe('loading using routes predefined via setupNamespaceLoader()', () => {