let { screenWidth } = wx.getSystemInfoSync();
ad = wx.createBannerAd({
  adUnitId: 'xxxxxxxxx',
  style: {
     left: 0,
     top: 0,
     width: 0,
     height: 0
  }
});

ad.onResize(() => {
  ad.style.width = screenWidth * 0.6;
  ad.style.left = (screenWidth - ad.style.realWidth) / 2;
  ad.style.top = screenHeight - ad.style.realHeight;
});