Using Realm with Expo Metro config "Exotic" bundling.

metro-transformer.js

const { createExoticTransformer } = require("@expo/metro-config/transformer");

module.exports = createExoticTransformer({
  // Add realm packages to the list of modules to transpile locally.
  transpileModules: ["@realm/", "realm"],
});

metro.config.js

// From docs: https://github.com/expo/expo-cli/tree/master/packages/metro-config
const { getDefaultConfig } = require("expo/metro-config");

const baseConfig = getDefaultConfig(__dirname, {
  // Initialize in exotic mode.
  // If you want to preserve `react-native` resolver main field, and omit cjs support, then leave this undefined
  // and skip setting the `EXPO_USE_EXOTIC` environment variable.
  mode: "exotic",
});

// Use the new transformer
baseConfig.transformer.babelTransformerPath = require.resolve(
  "./metro-transformer"
);

// Optionally, you can add support for the `react-native` resolver field back
// doing this will increase bundling time and size as many community packages ship untransformed code using this feature.
// Other packages like `nanoid` use the field to support `react-native` so you may need to enable it regardless.
// defaultConfig.resolver.resolverMainFields.unshift('react-native');

module.exports = baseConfig;