问electron要怎么拿到webContents.executeJavaScript执行之后的值？

app.on('ready', function() {
    mainWindow = new BrowserWindow({
        width: 1024,
        height: 1024
    });
    mainWindow.loadURL("https://www.baidu.com/");
    mainWindow.webContents.executeJavaScript(`
    var canvas = document.createElement('canvas')
    var imgDom = document.getElementById('cptImage_lab5_cpt')
    canvas.width = imgDom.width
    canvas.height = imgDom.height
    var ctx = canvas.getContext('2d')
    ctx.drawImage(imgDom,0,0,imgDom.width,imgDom.height)
    var ext = imgDom.src.substring(imgDom.src.lastIndexOf(".")+1).toLowerCase()
    var dataURL = canvas.toDataURL("image/"+ext)
    //我要怎么拿到这个dataURL的值`);
});