export default function App() {

    MQTT.createClient({
            uri: 'mqtt://test.mosquitto.org:1883',
            clientId: 'id1',
            //tls: false
        }).then(function(client) {
        
        client.on('closed', function() {
            console.log('mqtt.event.closed');
        });
        
        client.on('error', function(msg) {
            console.log('mqtt.event.error', msg);
        });
        
        client.on('message', function(msg) {
            console.log('mqtt.event.message', msg);
        });
        
        client.on('connect', function() {
            console.log('connected --------------------------->');
            client.subscribe('/data', 0);
            client.publish('/data', "test", 0, false);
        });
      
        client.connect();
        }).catch(function(err){
        console.log(err);
        }); 

    return (
        <View style={styles.container}>
            <Text>Hello World!!!!</Text>
            <StatusBar style="auto" />
        </View>
    );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
});

I imported MQTT this way:

import MQTT from 'sp-react-native-mqtt'; 

This library should be for react native and I copied the code from https://npmjs.com/package/sp-react-native-mqtt