Optimise/simplify getPlatformExtension

Summary:
`getPlatformExtension` extracts the `"ios"` from `"path/MyComponent.ios.js"`, if `"ios"` is a known platform. In the context of this function, this is only relevant to Haste.

Currently, the effective set of platforms is the union of those passed from Metro (from `config.resolver.platforms`) and a hardcoded set.

Entries in the hardcoded set but not in `config.resolver.platforms` are mostly superfluous because Metro will never resolve to them (although they would *prevent* `MyComponent.extraplatform` from being added to the Haste map with a generic platform).

The notable exception is `native`, which in Metro is a special platform alias used in the absence of a specific platform match.

Here, we remove the hardcoded set, and pass `native` from Metro when creating MetroFileMap.

Changelog: Internal

Reviewed By: motiz88

Differential Revision: D47050529

fbshipit-source-id: e9b5495e87133033a1f5270e36809c99eb75bbf9

packages/metro-file-map/src/index.js

@@ -364,7 +364,7 @@ export default class FileMap extends EventEmitter {
         this._startupPerfLogger?.point('constructHasteMap_start');
         const hasteOptions = {
           console: this._console,
-          platforms: this._options.platforms,
+          platforms: new Set(this._options.platforms),
           rootDir,
           throwOnModuleCollision: this._options.throwOnModuleCollision,
         };

packages/metro-file-map/src/lib/MutableHasteMap.js

@@ -34,7 +34,7 @@ const EMPTY_MAP: $ReadOnlyMap<string, DuplicatesSet> = new Map();
 
 type HasteMapOptions = $ReadOnly<{
   console?: ?Console,
-  platforms: $ReadOnlyArray<string>,
+  platforms: $ReadOnlySet<string>,
   rootDir: Path,
   throwOnModuleCollision: boolean,
 }>;
@@ -46,7 +46,7 @@ export default class MutableHasteMap implements HasteMap {
 
   +#console: ?Console;
   #throwOnModuleCollision: boolean;
-  +#platforms: $ReadOnlyArray<string>;
+  +#platforms: $ReadOnlySet<string>;
 
   constructor(options: HasteMapOptions) {
     this.#console = options.console ?? null;

packages/metro-file-map/src/lib/__tests__/getPlatformExtension-test.js

@@ -10,14 +10,16 @@
 
 import getPlatformExtension from '../getPlatformExtension';
 
+const PLATFORMS = new Set(['ios', 'android']);
+
 describe('getPlatformExtension', () => {
-  it('should get platform ext', () => {
-    expect(getPlatformExtension('a.ios.js')).toBe('ios');
-    expect(getPlatformExtension('a.android.js')).toBe('android');
-    expect(getPlatformExtension('/b/c/a.ios.js')).toBe('ios');
-    expect(getPlatformExtension('/b/c.android/a.ios.js')).toBe('ios');
-    expect(getPlatformExtension('/b/c/a@1.5x.ios.png')).toBe('ios');
-    expect(getPlatformExtension('/b/c/a@1.5x.lol.png')).toBe(null);
-    expect(getPlatformExtension('/b/c/a.lol.png')).toBe(null);
+  test('should get platform ext', () => {
+    expect(getPlatformExtension('a.ios.js', PLATFORMS)).toBe('ios');
+    expect(getPlatformExtension('a.android.js', PLATFORMS)).toBe('android');
+    expect(getPlatformExtension('c.android/a.ios.js', PLATFORMS)).toBe('ios');
+    expect(getPlatformExtension('/b/c/a.ios.js', PLATFORMS)).toBe('ios');
+    expect(getPlatformExtension('/b/c/a@1.5x.ios.png', PLATFORMS)).toBe('ios');
+    expect(getPlatformExtension('/b/c/a@1.5x.lol.png', PLATFORMS)).toBe(null);
+    expect(getPlatformExtension('/b/c/a.lol.png', PLATFORMS)).toBe(null);
   });
 });

packages/metro-file-map/src/lib/getPlatformExtension.js

@@ -8,23 +8,16 @@
  * @flow strict
  */
 
-const SUPPORTED_PLATFORM_EXTS = new Set(['android', 'ios', 'native', 'web']);
-
 // Extract platform extension: index.ios.js -> ios
 export default function getPlatformExtension(
   file: string,
-  platforms?: $ReadOnlyArray<string>,
+  platforms: $ReadOnlySet<string>,
 ): ?string {
   const last = file.lastIndexOf('.');
   const secondToLast = file.lastIndexOf('.', last - 1);
   if (secondToLast === -1) {
     return null;
   }
   const platform = file.substring(secondToLast + 1, last);
-  // If an overriding platform array is passed, check that first
-
-  if (platforms && platforms.indexOf(platform) !== -1) {
-    return platform;
-  }
-  return SUPPORTED_PLATFORM_EXTS.has(platform) ? platform : null;
+  return platforms.has(platform) ? platform : null;
 }

packages/metro/src/node-haste/DependencyGraph/createFileMap.js

@@ -96,7 +96,7 @@ function createFileMap(
     ignorePattern: getIgnorePattern(config),
     maxWorkers: config.maxWorkers,
     mocksPattern: '',
-    platforms: config.resolver.platforms,
+    platforms: [...config.resolver.platforms, MetroFileMap.H.NATIVE_PLATFORM],
     retainAllFiles: true,
     resetCache: config.resetCache,
     rootDir: config.projectRoot,