Error connecting to websockets NodeJS Express server

I have an express server with websockets configured:

/tmp/ws-test $ cat httpServer.js
const express = require('express');
const WebSocket = require('ws');

const app = express();
const port = 8080;

app.get('/', function(req, res, next) {
  return res.send('Hello World!');
});

const wss = new WebSocket.Server({ server: app });

wss.on('connection', function connection(ws) {
  ws.on('message', function incoming(message) {
    console.log('received: %s', message);
  });
  ws.send('something from server');
});

app.listen(port, function(err) {
  if (err) {
    throw err;
  }
  console.log(`listening on port ${port}!`);
});

It runs as expected:

/tmp/ws-test $ node httpServer.js
listening on port 8080!

I have a client:

/tmp/ws-test $ cat client.js
const WebSocket = require('ws');

const ws = new WebSocket('ws://localhost:8080');

ws.on('open', function open() {
  ws.send('something from client');
});

ws.on('message', function incoming(data) {
  console.log(`received: ${data}`);
});

It errors:

/tmp/ws-test $ node client.js
events.js:180
      throw er; // Unhandled 'error' event
      ^

Error: Unexpected server response: 200
    at ClientRequest.<anonymous> (/private/tmp/ws-test/node_modules/ws/lib/websocket.js:579:7)
    at ClientRequest.emit (events.js:203:13)
    at HTTPParser.parserOnIncomingClient [as onIncoming] (_http_client.js:575:27)
    at HTTPParser.parserOnHeadersComplete (_http_common.js:116:17)
    at Socket.socketOnData (_http_client.js:449:22)
    at Socket.emit (events.js:203:13)
    at addChunk (_stream_readable.js:295:12)
    at readableAddChunk (_stream_readable.js:276:11)
    at Socket.Readable.push (_stream_readable.js:210:10)
    at TCP.onStreamRead (internal/stream_base_commons.js:166:17)
Emitted 'error' event at:
    at abortHandshake (/private/tmp/ws-test/node_modules/ws/lib/websocket.js:697:15)
    at ClientRequest.<anonymous> (/private/tmp/ws-test/node_modules/ws/lib/websocket.js:579:7)
    [... lines matching original stack trace ...]
    at Socket.Readable.push (_stream_readable.js:210:10)

In the client I also tried using http for the web socket connection.

const ws = new WebSocket('http://localhost:8080');

But it throws the same error.

Why is the client erroring on connection to the server?

You should pass an http server instance to WebSocket.Server constructor instead of an instance of express application:

const express = require('express');
const WebSocket = require('ws');
const http = require('http');

const app = express();
const port = 8080;

const server = http.createServer(app);

app.get('/', function(req, res, next) {
    return res.send('Hello World!');
});

const wss = new WebSocket.Server({ server });

wss.on('connection', function connection(ws) {
    ws.on('message', function incoming(message) {
        console.log('received: %s', message);
    });
    ws.send('something from server');
});

server.listen(port, function(err) {
    if (err) {
        throw err;
    }
    console.log(`listening on port ${port}!`);
});