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// webview-preload.js
const { ipcRenderer } = require('electron')    

global.pingHost = () => {
  ipcRenderer.sendToHost('ping')
}

// webview-index.html:
<script>
  pingHost()
</script>

// window-index.html
<script>
  const webview = document.getElementById('mywebview')
  webview.addEventListener('ipc-message', event => {
    // prints "ping"
    console.log(event.channel)
  })
</script>