Diving into webpack module loader can often seem like a confusing and daunting task. Luckily, setting environment variables is rather straight forward.

At the top of your webpack.config.js file define your environment variables like so:

/**
 * Example Constants
 */
const ENV = process.env.ENV = process.env.NODE_ENV = 'development';
const API_URL = process.env.API_URL = 'http://localhost:3000';
const METADATA = {
  API_URL: API_URL,
  ENV: ENV
};

Then in the plugins array use DefinePlugin method to define global variables configured at compile time:

/**
 * Make Webpack Constants Available Globally
 */
   new webpack.DefinePlugin({
     'ENV': JSON.stringify(METADATA.ENV),
     'API_URL': JSON.stringify(METADATA.API_URL),
     'process.env': {
       'ENV': JSON.stringify(METADATA.ENV),
       'NODE_ENV': JSON.stringify(METADATA.ENV),
       'API_URL' : JSON.stringify(METADATA.API_URL),
     }
   }),

Re-compile and now your variables will be globally available.