RFC: Platform isAndroid, isIOS, isNative properties

proposals/0670-platform-is-platform-props.md

@@ -0,0 +1,82 @@
+---
+title: Platform isAndroid, isIOS, and isNative properties
+author:
+  - Ryan Linton
+date: 2023-06-14
+---
+
+# RFC0670: Platform isAndroid, isIOS, and isNative properties
+
+## Summary
+
+Add `isAndroid`, `isIOS`, and `isNative` to `Platform`.
+
+## Basic example
+
+```ts
+if (Platform.isAndroid) {
+  // do some Android only stuff here
+}
+```
+
+## Motivation
+
+Platform already defines `isPad`, `isTV`, and `isTesting`. In consulting and the companies I've worked at I've seen dozens (even hundreds) of apps, and nearly all backfill the missing ones `isAndroid` and `isIOS` with something along the lines of the following:
+
+```ts
+const isAndroid = Platform.OS === "android";
+const isIOS = Platform.OS === "ios";
+```
+
+`Platform` should provide these since so many devs want to express one-off platform checks this way.
+
+## Detailed design
+
+I did an initial implementation [here](https://github.com/facebook/react-native/pull/37870) but it has some drawbacks. As @NickGerleman [pointed out](https://github.com/facebook/react-native/pull/37870#pullrequestreview-1478897922) we don't want to maintain a centralized list of platforms. A better solution that keeps the platform definitions in their respective platforms is to update `Platform` to be a proxy object.
+
+```ts
+const Platform = new Proxy(
+  {
+    __constants: null,
+    OS: "android",
+    isAndroid: true,
+    isNative: true,
+    // ...
+  },
+  {
+    get(target, prop) {
+      if (prop in target) {
+        return target[prop];
+      }
+
+      if (prop.toString().startsWith("is")) {
+        return false;
+      }
+
+      return Reflect.get(...arguments);
+    },
+  }
+);
+```
+
+## Drawbacks
+
+- This change doesn't really offer any new functionality over conditional logic using `Platform.OS`. It's just a different (albeit quite common) way of expressing the same thing.
+- As @motiz88 [points out](https://github.com/facebook/react-native/pull/37870#issuecomment-1591225470) this breaks existing optimizations with Babel transforms that remove dead code based on `Platform.OS` conditionals. This proposal likely further complicates that by making `Platform` a proxy object.
+- All other platforms would need to update their `Platform` implementations to match.
+
+## Alternatives
+
+Another potential solution that doesn't use proxy but reads similarly would be to add an `is` method (e.g. `Platform.is('android')` or `Platform.is('ios')`). This may or may not have advantages considering the Babel transforms. It is less similar to the common code that inspired this proposal though.
+
+## Adoption strategy
+
+This is not a breaking change but would be a new API offering the same existing functionality provided by checking against `Platform.OS`. Existing React Native developers would upgrade to latest and discover via docs and types.
+
+## How we teach this
+
+Per motivation, React Native developers are already doing this. This just moves the definition of `isAndroid` and `isIOS` up into React Native.
+
+## Unresolved questions
+
+- Where are the Babel transforms for removing platform specific code and how do they work?