this

Arrow functions capture the this value of the enclosing context,

each()函数将元素作为第二个参数传递给回调.

但是更适合您的解决方案也是使用.map()而不是each()

let mediaBoxes = $(".now-thumbnail");
let titles = mediaBoxes.map((i,el) => {
  return {
    index: i,title: $(el).find(".now-thumbnail-bottomtext").text().trim()
  };
}).get();