How to detect platform on ionic stencil project?

I’m using a ionic-stencil-starter-pwa project and I need to detect the platform because I will build the app using appflow for ios and android and I need to do some things only in a certain platform.

I don’t use cordova or angular but vanilla javascript.

I’ve found this and I’ve installed it but I have doubts how to use it.

import { Device } from '@ionic-native/device';

export class myClass {
  device: typeof Device;

but if I do “npm run build” i have this error:

[ ERROR ]  TypeScript: ./node_modules/@ionic-native/device/index.d.ts:1:35
           Cannot find module '@ionic-native/core' or its corresponding type declarations.

      L1:  import { IonicNativePlugin } from '@ionic-native/core';
      L2:  /**

I don’t really understand why this problem appears because I’ve installed the plugin like the documentation says