Parameterized use of TFunction fails while WithT use works (#1188)

* Parameterized use of TFunction fails while WithT use works

* rename tests

* use simple type declaration for TFunction - allows for external parameterized use

* fix lint error and add new test with optional arg

* Using typescript scrict, it appears undefined must be in the TResult to play nice with optional args/props

index.d.ts

@@ -471,7 +471,7 @@ declare namespace i18next {
     // Expose parameterized t in the i18next interface hierarchy
     // NOTE: duplicate of TFunction - not sure why I can't find a workable reuse pattern
     t<
-      TResult extends string | object | Array<string | object> = string,
+      TResult extends string | object | Array<string | object> | undefined = string,
       TKeys extends string = string,
       TValues extends object = object
     >(
@@ -480,14 +480,15 @@ declare namespace i18next {
     ): TResult;
   }
 
-  // NOTE: duplicate of WithT.t - not sure why I can't find a workable reuse pattern
-  interface TFunction<
-    TResult extends string | object | Array<string | object> = string,
+  // NOTE: somewhat duplicate of WithT.t but cannot use interface - not sure why I can't find a workable reuse pattern
+  type TFunction = <
+    TResult extends string | object | Array<string | object> | undefined = string,
     TKeys extends string = string,
     TValues extends object = object
-  > {
-    (key: TKeys | TKeys[], options?: TOptions<TValues>): TResult;
-  }
+  >(
+    key: TKeys | TKeys[],
+    options?: TOptions<TValues>,
+  ) => TResult;
 
   interface Resource {
     [language: string]: ResourceLanguage;

package.json

@@ -73,7 +73,7 @@
     "rollup-plugin-uglify": "1.0.1",
     "sinon": "1.17.7",
     "tslint": "^5.12.1",
-    "typescript": "^3.2.2",
+    "typescript": "^3.2.4",
     "watchify": "3.9.0",
     "yargs": "6.6.0"
   },

test/typescript/t.tfunction.test.ts

@@ -0,0 +1,35 @@
+import i18next from 'i18next';
+
+/**
+ * Use of the exported TFunction in external utility methods such as
+ *  NamespaceConsumer children t
+ */
+function childrenNamespacesConsumer(t: i18next.TFunction, i18n: i18next.i18n) {
+  // sanity first - tests from i18next t.test
+  const is: string = i18n.t('friend'); // same as <string>
+  const io: object = i18n.t<object>('friend');
+  const isa: string[] = i18n.t<string[]>('friend');
+  const ioa: object[] = i18n.t<object[]>('friend');
+
+  // (failing) now try t provided by NamespacesConsumer
+  const s: string = t('friend'); // same as <string>
+  const o: object = t<object>('friend');
+  const sa: string[] = t<string[]>('friend');
+  const oa: object[] = t<object[]>('friend');
+}
+
+function callsMethodWithOptionalArg(t: i18next.TFunction, i18n: i18next.i18n) {
+  function displayHint(hint?: string) {
+    return String(hint);
+  }
+  displayHint(i18n.t('friend'));
+  displayHint(t('friend'));
+}
+
+function callsMethodWithRequiredArg(t: i18next.TFunction, i18n: i18next.i18n) {
+  function displayHint(hint: string) {
+    return String(hint);
+  }
+  displayHint(i18n.t('friend'));
+  displayHint(t('friend'));
+}