socket.io 오류 ReferenceError: socket is not defined

const express = require('express'); //express 불러옴
const app = express();
app.use(express.urlencoded({ extended: true }));
const http = require('http').createServer(app);
const { Server } = require("socket.io");
const io = new Server(http);
const MongoClient = require('mongodb').MongoClient;
const methodOverride = require('method-override');
app.use(methodOverride('_method'));
app.set('view engine', 'ejs');
require('dotenv').config()

MongoClient.connect(process.env.DB_URL, (에러, client) => {
    //연결되면 할일.
    if (에러) { return console.log(에러); }
    db = client.db('nodetutorial');
    http.listen(process.env.PORT, function () { //soket.io 사용하기위해 app.listen -> http.listen으로 변경
        console.log('db연결 성공');
    });
})

io.on('connection',function(){
    console.log('연결되었어요');    
    socket.on('인삿말', function(data){
        console.log(data)
      });
});
app.get('/chat', function (요청, 응답) {
    응답.render('chat.ejs');
});

말씀하신대로 더 윗부분으로 올렸으나 똑같이

ReferenceError: socket is not defined 라고 나오네요 ㅜㅜ