firefox addone SDK：基本扩展：什么是{}？

Question

var { ActionButton } = require("sdk/ui/button/action");

{}的内容是什么？

看起来面板不需要

var panel = require("sdk/panel").Panel({
  width: 180,
  height: 180,
  contentURL: "https://en.wikipedia.org/w/index.php?title=Jetpack&useformat=mobile"
});