Updated working RealmDB example

App.js

import React from 'react';
import {View, Text, StyleSheet, Button, Alert} from 'react-native';
import Realm from 'realm';
const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
  },
  welcome: {
    backgroundColor: 'cyan',
  },
});
const DogSchema = {name: 'Dog', properties: {name: 'string'}};
const realm = new Realm({
  schema: [DogSchema],
  deleteRealmIfMigrationNeeded: true,
});
const App = () => {
  const [state, setState] = React.useState({});

  React.useEffect(() => {
    realm.write(() => {
      realm.create('Dog', {name: 'Rex'});
    });
    setState({realm});
  }, []);

  function handleCloseDatabase() {
    console.log(realm.objects('Dog').length);
    realm.close();
    Alert.alert('DB Closed.');

    return;
  }
  console.log(realm);
  const info = state.realm
    ? 'Number of dogs in this Realm: ' + state.realm.objects('Dog').length
    : 'Loading...';
  console.log(info);
  return (
    <View style={styles.container}>
      <Text style={styles.welcome}>{info}</Text>
      <Button title="Close Database" onPress={handleCloseDatabase} />
    </View>
  );
};
export default App;