Can't require() default export value in Babel 6.x

TL;DR

You have to use

const app = require('./app').default;
app();

Explanation

Babel 5 used to have a compatibility hack for export default: if a module contained only one export, and it was a default export, it was assigned to module.exports. So, for example, your module app.js

export default function () {}

would be transpiled to this

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});

exports["default"] = function () {};

module.exports = exports["default"];

This was done purely for compatibility with require-ing Babel-transpiled modules (like you are doing). It was also inconsistent; if a module contained both named and default exports, it could not be require-d.

In reality, according to the ES6 module spec, a default export is no different than a named export with the name default. It is just syntactic sugar which can be statically resolved at compile time, so this

import something from './app';

is the same as this

import { default as something } from './app';

That being said, it appears that Babel 6 decided to drop the interoperability hack when transpiling modules. Now, your module app.js is transpiled as

'use strict';

Object.defineProperty(exports, "__esModule", {
    value: true
});

exports.default = function () {};

As you see, no more assignment to module.exports. To require this module, you need to do

const app = require('./app').default;
app();

Or, more concisely, and closer to your original code:

require('./app').default();

Just to follow up with the correct answer above.

If you wanna use default export behavior of babel@5, you can try babel-plugin-add-module-exports plugin.

It's working pretty well for me.

If this doesn't work

require('./app').default()

use

require('./app').default

Without the function call at the end.