typeof一个模块都是undefined，new构造函数提示not defined

模块`lib/socket-connection.js`：
``` js
module.exports = function SocketConnection () {}
```
引入：
``` js
import { SocketConnection } from 'xxx'

console.log(typeof SocketConnection) // undefined
console.log(SocketConnection) // function SocketConnection () {}

SocketConnection() // 正常执行函数
new SocketConnection() // Uncaught ReferenceError: SocketConnection is not defined
```