electron/spec-main/fixtures/api/webrequest.html

<script>
  var url = new URL(location.href)
  const port = new URLSearchParams(url.search).get("port")
  const ipcRenderer = require('electron').ipcRenderer
  let count = 0
  function checkFinish() {
    count++
    if (count === 2) {
      ipcRenderer.send('websocket-success')
    }
  }

  var conn = new WebSocket(`ws://127.0.0.1:${port}/websocket`)
  conn.onopen = data => conn.send('foo')
  conn.onmessage = wsMsg
  function wsMsg(msg) {
    if (msg.data === 'bar') {
      checkFinish()
    } else {
      ipcRenderer.send('fail')
    }
  }

  fetch(`http://127.0.0.1:${port}/`).then(() => {
    checkFinish()
  })
</script>