Splitting locale matching into separate file to facilitate testing

src/components/LocaleSwitcher/index.js

@@ -5,22 +5,9 @@ import ExpandIcon from 'react-feather/dist/icons/chevrons-down';
 
 import i18n, { locales } from 'i18n';
 
+import localeToAvailable from './locale-to-available';
 import style from './style.module.css';
 
-const localeToAvailable = (locale, available, defaultLocale) => {
-  if (available.includes(locale)) {
-    return locale;
-  }
-
-  const parts = locale.split('-');
-
-  if (parts.length > 0 && available.includes(parts[0])) {
-    return parts[0];
-  }
-
-  return defaultLocale;
-};
-
 export class LocaleSwitcher extends React.PureComponent {
   state = {
     current: localeToAvailable(

src/components/LocaleSwitcher/locale-to-available.js

@@ -0,0 +1,15 @@
+const localeToAvailable = (locale, available, defaultLocale) => {
+  if (available.includes(locale)) {
+    return locale;
+  }
+
+  const parts = locale.split('-');
+
+  if (parts.length > 0 && available.includes(parts[0])) {
+    return parts[0];
+  }
+
+  return defaultLocale;
+};
+
+export default localeToAvailable;

src/components/LocaleSwitcher/locale-to-available.test.js

@@ -0,0 +1,18 @@
+import localeToAvailable from './locale-to-available';
+
+describe('localeToAvailable', () => {
+  test('when requested language and region are available', () => {
+    expect(localeToAvailable('en-US', ['en', 'en-US', 'other'], 'other'))
+      .toEqual('en-US');
+  });
+
+  test('when only requested language is available', () => {
+    expect(localeToAvailable('en-US', ['en', 'en-GB', 'other'], 'other'))
+      .toEqual('en');
+  });
+
+  test('when language is unavailable', () => {
+    expect(localeToAvailable('en-US', ['tlh', 'other'], 'other'))
+      .toEqual('other');
+  });
+});