install socket io

npm i socket.io -S

index.js

const express = require('express');
const Http = require('http');  // new!
const socketIo = require('socket.io');  // new!

// ...

const app = express();
const http = Http.createServer(app);  // new!
const io = socketIo(http);  // new!

// ...

io.on('connection', (socket) => {
  console.log("someone connected!");

	socket.emit("BUY_GOODS", {
    nickname: '서버가 보내준 구매자 닉네임',
    goodsId: 10, // 서버가 보내준 상품 데이터 고유 ID
    goodsName: '서버가 보내준 구매자가 구매한 상품 이름',
    date: '서버가 보내준 구매 일시'
  });

  socket.on('disconnect', () => {
    console.log("someone disconnected!");
  })
});

// ...

http.listen(8080, () => {
    console.log("listening at <http://localhost:8080>");
});

Check if front-end gives "BUY" event well

io.on('connection', (socket) => {
  console.log("someone connected!");

  socket.on("BUY", (data) => {  // when someone clicked "BUY" button
    console.log("client bought ", data);
  })

  socket.on('disconnect', () => {
    console.log("someone disconnected!");
  })
});

Send every connected client BUY event!

• To send EVERYONE

io.on('connection', (socket) => {
  console.log("someone connected!");

  socket.on("BUY", (data) => {
    const payload = {
      nickname: data.nickname,
      goodsId: data.goodsId,
      goodsName: data.goodsName,
      date: new Date().toISOString(),
    }

    console.log("client bought ", data);

    io.emit("BUY_GOODS", payload);
  })

  socket.on('disconnect', () => {
    console.log("someone disconnected!");
  })
});

• To send everyone EXCEPT FOR ME

io.on('connection', (socket) => {
  console.log("someone connected!");

  socket.on("BUY", (data) => {
    const payload = {
      nickname: data.nickname,
      goodsId: data.goodsId,
      goodsName: data.goodsName,
      date: new Date().toISOString(),
    }

    console.log("client bought ", data);

    socket.broadcast.emit("BUY_GOODS", payload);  // send everyone except for me
  })

  socket.on('disconnect', () => {
    console.log("someone disconnected!");
  })
});