hexo.extend.filter.register("before_post_render", function(data) {
  if (data.cover == false) {
    data.randomcover = random_cover();
    return data;
  }
  data.cover = data.cover || random_cover();
  return data;
});

var random_cover = function() {
  var cover;
  var num;
  if (!Array.isArray(hexo.theme.config.cover.default_cover)) {
    cover = hexo.theme.config.cover.default_cover;
    return cover;
  } else {
    num = Math.floor(
      Math.random() * hexo.theme.config.cover.default_cover.length
    );
    cover = hexo.theme.config.cover.default_cover[num];
    return cover;
  }
};