Example of basic Webpack config for a simple website. Guide: here

Install all packages:

$ npm install

Run webpack

$ npm run build

Done! Open index.html in browser for a cat image.

In postcss.config.js there is a commented out check for process.env.NODE_ENV variable. Keep in mind that even if you set Webpack mode to production it won't automatically change Node environment variable.

The simplest way to configure this is to install cross-env package:

$ npm install --save-dev cross-env

Then just add another npm script in package.json for production mode:

"scripts": {
  "build": "webpack --config webpack.config.js",
  "build-production": "cross-env NODE_ENV=production webpack --config webpack.config.js"
}

Now when you run npm run build-production the process.env.NODE_ENV variable will be production and postcss.config.js check is going to work:

if (process.env.NODE_ENV === 'production') {
    module.exports = {
        plugins: [
            require('autoprefixer'),
            require('cssnano')
        ]
    }
}

From Webpack documentation: Technically, NODE_ENV is a system environment variable that Node.js exposes into running scripts. It is used by convention to determine dev-vs-prod behavior by server tools, build scripts, and client-side libraries. Contrary to expectations, process.env.NODE_ENV is not set to "production" within the build script webpack.config.js. Thus, conditionals like process.env.NODE_ENV === 'production' ? '[name].[hash].bundle.js' : '[name].bundle.js' within webpack configurations do not work as expected.