import { Plugins, PluginResultError } from '@capacitor/core';

const { FaceId } = Plugins;

...
// check if device supports Face ID or Touch ID
FaceId.isAvailable().then(checkResult => {
  if(checkResult.value) {
    FaceId.auth().then(() => {
      console.log('authenticated');
    }).catch((error: PluginResultError) => {
      // handle rejection errors
      console.error(error.message);
    });
  } else {
    // use custom fallback authentication here
  }
});