Share code between iOS and Android

device.lucent.ts

import { PLATFORM } from "lucent:platform";
import { UIDevice } from "lucent:ios/UIKit";
import { Build, Build_VERSION } from "lucent:android/android.os";
import { main } from "lucent:thread";

export type DeviceInfo = { platform: string; model: string; label: string };

// Shared code: plain Lucent.
function label(model: string, version: string): string {
  return `${model} (${version})`;
}

export async function deviceInfo(): Promise<DeviceInfo> {
  if (PLATFORM === "ios") {
    const [model, version] = await main(() => [
      UIDevice.current.model,
      UIDevice.current.systemVersion,
    ]);
    return { platform: PLATFORM, model, label: label(model, `iOS ${version}`) };
  } else {
    const model = Build.MODEL ?? "Android";
    return {
      platform: PLATFORM,
      model,
      label: label(model, `Android ${Build_VERSION.RELEASE ?? ""}`),
    };
  }
}

• if (PLATFORM === "ios") { … } else { … } compiles each branch for its own platform. ? :, switch (PLATFORM) and PLATFORM === "ios" && … work too.
• A top-level function, class or variable that uses one platform's SDK outside a branch belongs to that platform, and compiles only there.
• Exports run on both platforms, so they branch inside.
• Both branches are type-checked on every build. Where a platform's SDK isn't installed, its code is untyped and skipped.

A guard clause works too, as TypeScript narrows it: after if (PLATFORM === "ios") return …; the rest of the block is Android code. The test must be PLATFORM alone (not PLATFORM === "ios" && ready), with no else, and its branch must end in return, throw, break or continue.

When to split into platform files

When the two halves share nothing, split the module: x.lucent.ts holds only export declare functions and types, and x.ios.lucent.ts and x.android.lucent.ts each implement all of them (LUCENT3005). JavaScript imports x.lucent either way.