如何将回调函数结果值分配给新变量？

Question

我想从电子对话框的回调函数中获取值，并在新变量中重新分配回调结果。

const {dialog } = electron;
let options = {
  type: 'warning',
  buttons: ['Close','Register'],
  message: 'license Key Not Found!',
  detail: "license Key does't found! Please register to download license key."
};
var results = dialog.showMessageBox(null,options, function (res) {
  return res;
});
console.log(results);

出现未定义的值。