example for extend ionic/app-scripts/config/webpack.config.js

webpack.config.js

// don't forget to install "lodash" "dotenv" 

var ionicWebpackConfig = require('@ionic/app-scripts/config/webpack.config.js');
var webpack = require('webpack');
var _ = require('lodash');
var dotenvConfig = require('dotenv').config();
var dev = ionicWebpackConfig.dev;
var prod = ionicWebpackConfig.prod;

var loadProcessEnvValues = function() {
  return new webpack.DefinePlugin({
    'process.env': _(process.env)
                    .pick(_.keys(dotenvConfig.parsed))
                    .mapValues((v) => (JSON.stringify(v)))
                    .value()
  });
};

dev.plugins.push(loadProcessEnvValues());
prod.plugins.push(loadProcessEnvValues());

module.exports = {
  dev: dev,
  prod: prod
}

cool, thanks for this.

As a reference after adapted the gist
(we use awesome-typescript-loader as TS loader)

var webpack = require("webpack");
var dotenvConfig = require("dotenv").config().parsed;
var _ = require("lodash");
var TsConfigPathsPlugin = require("awesome-typescript-loader")
  .TsConfigPathsPlugin;

var ionicWebpackConfig = require("@ionic/app-scripts/config/webpack.config.js");
var dev = ionicWebpackConfig.dev;
var prod = ionicWebpackConfig.prod;

var processEnv = {
  "process.env": _(process.env)
    .pick(_.union(_.keys(dotenvConfig), ["IONIC_ENV"]))
    .mapValues(v => JSON.stringify(v))
    .value()
};

var createDotEnvPlugin = function() {
  return new webpack.DefinePlugin(processEnv);
};
var createTsConfigPathsPlugin = function() {
  return new TsConfigPathsPlugin({
    configFilePath: "..",
    compiler: "typescript"
  });
};

dev.plugins.unshift(createDotEnvPlugin());
dev.resolve.plugins = [createTsConfigPathsPlugin()];

prod.plugins.unshift(createDotEnvPlugin());
prod.resolve.plugins = [createTsConfigPathsPlugin()];

module.exports = {
  dev: dev,
  prod: prod
};

Cool, thank you!