fileA.js
import { bThing } from './fileB'
console.log(bThing)
export const aThing = 'aThing'
fileB.js
import { aThing } from './fileA'
console.log(aThing)
export const bThing = 'bThing'
fileB中的console.log输出undefined。如果我在fileA中注释掉导入，那么fileB中的导入将起作用！