Merge pull request #27904 from LinusU/react-navigation-fallback

[react-navigation] Fix return type when fallback provided

types/react-navigation/index.d.ts

@@ -18,6 +18,7 @@
 //                 Jérémy Magrin <https://github.com/magrinj>
 //                 Luca Campana <https://github.com/TizioFittizio>
 //                 Ullrich Schaefer <https://github.com/stigi>
+//                 Linus Unnebäck <https://github.com/LinusU>
 //                 Yosuke Seki <https://github.com/jshosomichi>
 //                 Jake <https://github.com/jakebooyah>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
@@ -588,7 +589,8 @@ export interface NavigationScreenProp<S, P = NavigationParams> {
   openDrawer: () => any;
   closeDrawer: () => any;
   toggleDrawer: () => any;
-  getParam: <T extends keyof P>(param: T, fallback?: P[T]) => P[T];
+  getParam<T extends keyof P>(param: T, fallback: NonNullable<P[T]>): NonNullable<P[T]>;
+  getParam<T extends keyof P>(param: T): P[T];
   setParams: (newParams: Partial<P>) => boolean;
   addListener: (
     eventName: 'willBlur' | 'willFocus' | 'didFocus' | 'didBlur',

types/react-navigation/react-navigation-tests.tsx

@@ -497,6 +497,7 @@ const CustomHeaderStack = createStackNavigator({
 
 interface ScreenProps {
     name: string;
+    optionalAge?: number;
     onPlay(): void;
 }
 
@@ -513,9 +514,16 @@ class SetParamsTest extends React.Component<NavigationScreenProps<ScreenProps>>
 
     render() {
         const name = this.props.navigation.getParam('name');
+        const age = this.props.navigation.getParam('optionalAge');
+
+        // $ExpectType number | undefined
+        this.props.navigation.getParam('optionalAge');
+
+        // $ExpectType number
+        this.props.navigation.getParam('optionalAge', 0);
 
         return (
-            <Text>My name is {name}</Text>
+            <Text>My name is {name} and I am {age} years old.</Text>
         );
     }
 }