App.js

import React, { Component } from 'react';
import {
    Platform,
    StyleSheet,
    Text,
    View
} from 'react-native';
const Web3 = require('web3');
const web3 = new Web3();
web3.setProvider(new web3.providers.HttpProvider('https://ropsten.infura.io/v3/e69df96932bd4e9db7451fab8d6e0c85'));


const instructions = Platform.select({
  ios: 'Press Cmd+R to reload,\n' + 'Cmd+D or shake for dev menu',
  android:
    'Double tap R on your keyboard to reload,\n' +
    'Shake or press menu button for dev menu',
});

type Props = {};



export default class App extends Component<Props> {

    constructor(props) {
        super(props);
        this.state = {
            balance: 0,
        }
    }

    componentDidMount(){
        web3.eth.getBalance('0xcFA84f1a15fC411CcFD8b6bb102cC9613b93188d', (err, balance) => {
            console.log('balance ' + balance);
            this.setState({balance});
        });
    }

    render() {
        return (
            <View style={styles.container}>
                <Text style={styles.welcome}>
                    Balance of 0xcFA84f1a15fC411CcFD8b6bb102cC9613b93188d: ({this.state.balance/10e17} rETH)
                </Text>
                <Text style={styles.instructions}>
                    {instructions}
                </Text>
            </View>
        );
    }

}

const styles = StyleSheet.create({
    container: {
        flex: 1,
        justifyContent: 'center',
        alignItems: 'center',
        backgroundColor: '#F5FCFF',
    },
    welcome: {
        fontSize: 20,
        textAlign: 'center',
        margin: 10,
    },
    instructions: {
        textAlign: 'center',
        color: '#333333',
        marginBottom: 5,
    },
});