electron/spec/fixtures/pages/window-open-postMessage-driver.html

<script>
  const child = window.open(`./window-open-postMessage.html`, '', 'show=no,nodeIntegration=yes,contextIsolation=no')
  window.onmessage = (e) => {
    if (e.data === 'ready') {
      child.postMessage('testing', '*')
    } else {
      require('electron').ipcRenderer.send('complete', {eventOrigin: e.origin, ...JSON.parse(e.data)})
    }
  }
</script>