Yet another realization of elm-app start script

start.js

const pathExists = require('path-exists');
const chalk = require('chalk');
const webpack = require('webpack');
const WebpackDevServer = require('webpack-dev-server');
const config = require('../config/webpack.config.dev');
const formatWebpackMessages = require('react-dev-utils/formatWebpackMessages');
const clearConsole = require('react-dev-utils/clearConsole');
const openBrowser = require('react-dev-utils/openBrowser');
const detect = require('detect-port');

const DEFAULT_PORT = 3000;

process.env.NODE_ENV = 'development';

if (pathExists.sync('elm-package.json') === false) {
  console.log('Please, run the build script from project root directory');
  process.exit(0);
}

detect(DEFAULT_PORT)
  // Configure compiler
  .then(function(unoccupiedPort) {
    // http://webpack.github.io/docs/node.js-api.html#the-long-way
    const compiler = webpack(config);

    compiler.plugin('invalid', compilingInProgress);
    compiler.plugin('done', function (stats) {      
      if (stats.hasErrors() || stats.hasWarnings()) {
        compilationError(stats);
      } else {
        compilationDone(unoccupiedPort);
      }
    });
    
    return {
      port: unoccupiedPort,
      compiler: compiler
    };
  })
  // Launch WebpackDevServer.
  .then(function (config) {
    const devServer = new WebpackDevServer(config.compiler, {
      hot: true,
      inline: true,
      publicPath: '/',
      quiet: true
    });

    devServer.listen(config.port, function (err) {
      if (err) {
        return console.log(err);
      }
    });
  
    return config.port;
  })
  .then(function (port) {
    openBrowser('http://localhost:' + port + '/');
  })
  .catch(function (err) {
    console.log(err);
  });


function compilingInProgress() {
  clearConsole();
  
  console.log('Compiling...');
}

function compilationDone(port) {
  clearConsole();
  
  console.log(chalk.green('Compiled successfully!'));
  
  if (DEFAULT_PORT !== port) {
    console.log(chalk.yellow('Default port ' + DEFAULT_PORT + ' is already busy, we will use ' + unoccupiedPort));
  }
  console.log('\nThe app is running at:');
  console.log('\n    ' + chalk.cyan('http://localhost:' + unoccupiedPort + '/'));
  console.log('\nTo create production build, run:');
  console.log('\n    elm-app build');
}

function compilationError(stats) {
  clearConsole();
  
  console.log(chalk.red('Compilation failed.\n'));
  
  const json = formatWebpackMessages(stats.toJson({}, true));
  json.errors.forEach(function (message) {
    console.log(message);
    console.log();
  });
}