Improve null type checking of Platform.select (#41455)

types/react-native/index.d.ts

@@ -5569,7 +5569,8 @@ interface PlatformStatic {
     /**
      * @see https://facebook.github.io/react-native/docs/platform-specific-code.html#content
      */
-    select<T>(specifics: { [platform in PlatformOSType | 'default']?: T }): T;
+    select<T>(specifics: ({ [platform in PlatformOSType]?: T } & { default: T }) | ({ [platform in PlatformOSType]: T })): T;
+    select<T>(specifics: { [platform in PlatformOSType]?: T }): T | undefined;
 }
 
 interface PlatformIOSStatic extends PlatformStatic {

types/react-native/test/index.tsx

@@ -21,6 +21,7 @@ import {
     CheckBox,
     ColorPropType,
     DataSourceAssetCallback,
+    DeviceEventEmitter,
     DeviceEventEmitterStatic,
     Dimensions,
     Image,
@@ -242,7 +243,7 @@ const combinedStyle5: StyleProp<TextStyle> = StyleSheet.compose(
     Math.random() < 0.5 ? composeTextStyle : null,
 );
 
-const combinedStyle6: StyleProp<TextStyle> = StyleSheet.compose(
+const combinedStyle6: StyleProp<TextStyle | null> = StyleSheet.compose(
     null,
     null,
 );
@@ -438,7 +439,7 @@ export class SectionListTest extends React.Component<SectionListProps<string>, {
     }
 
     scrollMe = () => {
-        this.myList.current.scrollToLocation({ itemIndex: 0, sectionIndex: 1 });
+        this.myList.current && this.myList.current.scrollToLocation({ itemIndex: 0, sectionIndex: 1 });
     };
 
     render() {
@@ -661,7 +662,7 @@ const dataSourceAssetCallback1: DataSourceAssetCallback = {
 const dataSourceAssetCallback2: DataSourceAssetCallback = {};
 
 // DeviceEventEmitterStatic
-const deviceEventEmitterStatic: DeviceEventEmitterStatic = null;
+const deviceEventEmitterStatic: DeviceEventEmitterStatic = DeviceEventEmitter;
 deviceEventEmitterStatic.addListener("keyboardWillShow", data => true);
 deviceEventEmitterStatic.addListener("keyboardWillShow", data => true, {});
 
@@ -726,7 +727,7 @@ class TextInputTest extends React.Component<{}, { username: string }> {
     render() {
         return (
             <View>
-                <Text onPress={() => this.username.focus()}>Username</Text>
+                <Text onPress={() => this.username && this.username.focus()}>Username</Text>
 
                 <TextInput
                     ref={input => (this.username = input)}
@@ -776,7 +777,7 @@ export class ImageTest extends React.Component {
         const image: ImageResolvedAssetSource = Image.resolveAssetSource({ uri });
         console.log(image.width, image.height, image.scale, image.uri);
 
-        Image.queryCache([uri]).then(({ [uri]: status }) => {
+        Image.queryCache && Image.queryCache([uri]).then(({ [uri]: status }) => {
             if (status === undefined) {
                 console.log("Image is not in cache");
             } else {
@@ -985,6 +986,11 @@ const PlatformTest = () => {
     }
 };
 
+Platform.select({ android: 1 }); // $ExpectType number | undefined
+Platform.select({ android: 1, ios: 2, default: 0 }); // $ExpectType number
+Platform.select({ android: 1, ios: 2, macos: 3, web: 4, windows: 5 }); // $ExpectType number
+Platform.select({ android: 1, ios: 2, macos: 3, web: 4, windows: 5, default: 0 }); // $ExpectType number
+
 // ProgressBarAndroid
 const ProgressBarAndroidTest = () => {
     <ProgressBarAndroid

types/react-native/tsconfig.json

@@ -4,7 +4,7 @@
         "lib": ["es6"],
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "strictFunctionTypes": true,
         "baseUrl": "../",
         "jsx": "react",