webpack issue 的回答：https://github.com/webpack/webpack/issues/1114

CommonJs spec defines only ​​exports​​​. But ​​module.exports​​​ is used by node.js and many other CommonJs implementations.
commonjs mean pure CommonJs
commonjs2 also includes the ​​​module.exports​​ stuff.

CommonJS 只有 export
CommonJS2 有 export 和 module.export