examples/with-socket-io/app/App.js

import * as React from "react";
import { StyleSheet, Text, View } from "react-native";

const io = require("socket.io-client");

// Replace this URL with your own, if you want to run the backend locally!
const SocketEndpoint = "https://socket-io-expo-backend-dtyxsdtzxb.now.sh";

export default class App extends React.Component {
  state = {
    isConnected: false,
    data: null,
  };
  componentDidMount() {
    const socket = io(SocketEndpoint, {
      transports: ["websocket"],
    });

    socket.on("connect", () => {
      this.setState({ isConnected: true });
    });

    socket.on("ping", (data) => {
      this.setState(data);
    });
  }

  render() {
    return (
      <View style={styles.container}>
        <Text>connected: {this.state.isConnected ? "true" : "false"}</Text>
        {this.state.data && <Text>ping response: {this.state.data}</Text>}
      </View>
    );
  }
}

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