webpack.config.js

const webpack = require('webpack')
const HtmlWebpackPlugin = require('html-webpack-plugin')

module.exports = {
  ...

  plugins: [
    new HtmlWebpackPlugin({
      template: 'src/index.html',
      inject: true
    }),
    new webpack.DefinePlugin({
      __CLIENT_ID__: JSON.stringify(process.env.CLIENT_ID || ''),
      __REDIRECT_URI__: JSON.stringify(process.env.REDIRECT_URI || 'http://localhost:8080/callback')
    }),
    new webpack.ProvidePlugin({
      'fetch': 'imports?this=>global!exports?global.fetch!whatwg-fetch'
    })
  ]
}