Configure @alias in Ionic 3 with TypeScript. This is done also to configure environments.

tsconfig.json

{
  "compilerOptions": {
    "paths": {
      "@src/*": ["./*"],
      "@app/*": ["./app/*"],
      "@pages/*": ["./pages/*"],
      "@env": [ "environments/environment" ]
    }
  },

webpack.config.js

var chalk = require("chalk");
const path = require('path');
const useDefaultConfig = require('@ionic/app-scripts/config/webpack.config.js');
const fs = require('fs');

const env = process.env.IONIC_ENV;

console.log('ENV is set to', env);

useDefaultConfig[env].resolve.alias = {
  "@src": path.resolve('./src/'),
  "@app": path.resolve('./src/app/'),
  "@pages": path.resolve('./src/pages/'),
  "@env": path.resolve(environmentPath(env))
};

if (env !== 'prod' && env !== 'dev') {
  // Default to dev config
  useDefaultConfig[env].resolve.alias = {
    "@src": path.resolve('./src/'),
    "@app": path.resolve('./src/app/'),
    "@pages": path.resolve('./src/pages/'),
    "@env": path.resolve(environmentPath(env))
  };
}

function environmentPath(env) {
  var filePath = './src/environments/environment' + (env === 'prod' ? `.${env}` : '') + '.ts';
  if (!fs.existsSync(filePath)) {
    console.log(chalk.red('\n' + filePath + ' does not exist!'));
  } else {
    return filePath;
  }
}

module.exports = function() {
  return useDefaultConfig;
};