// webpack.config.js
module.exports = {
    ...
    target: 'node'
    ...
}

// webpack.config.js
module.exports = {
    ...
    plugins: [new webpack.HotModuleReplacementPlugin()]
    ...
};

// webpack.config.js
module.exports = {
    ...
    entry: [
        'webpack/hot/poll?1000', // This differs from client side HMR
        'app.js'
    ]
    ...
};

// app.js
let config = require('./config');
setInterval(() => {
    console.log(config.FOO_BAR);
}, 2000);

// NOTE: This would need to be done everywhere you use './config' so you might want to create a loader to generate it.
if (module.hot) {
    module.hot.accept('./config', () => {
        let config = require('./config');
    });
}