External parameters from webpack to code

index.js

// __PROCESS__ is object from webpack, see "webpack.config.js"
const parameter = __PROCESS__.parameter;

...

webpack.config.js

// regular webpack configuration
module.exports = {
  plugins: [
    new webpack.DefinePlugin({
      '__PROCESS__': {
        'parameter': `'${process.env.PARAM}'`, // here we pass enviromental parameter to code
      }
    })
  ]
  
  ...
};