create-react-app/packages/react-scripts/scripts/utils/createWebpackCompiler.js

var chalk = require('chalk');
var webpack = require('webpack');
var clearConsole = require('react-dev-utils/clearConsole');
var formatWebpackMessages = require('react-dev-utils/formatWebpackMessages');

var isInteractive = process.stdout.isTTY;
var handleCompile;

// You can safely remove this after ejecting.
// We only use this block for testing of Create React App itself:
var isSmokeTest = process.argv.some(arg => arg.indexOf('--smoke-test') > -1);
if (isSmokeTest) {
  handleCompile = function (err, stats) {
    if (err || stats.hasErrors() || stats.hasWarnings()) {
      process.exit(1);
    } else {
      process.exit(0);
    }
  };
}

module.exports = function createWebpackCompiler(config, onReadyCallback) {
  // "Compiler" is a low-level interface to Webpack.
  // It lets us listen to some events and provide our own custom messages.
  try {
    var compiler = webpack(config, handleCompile);
  } catch (err) {
    console.log(chalk.red('Failed to compile.'));
    console.log();
    console.log(err.message || err);
    console.log();
    process.exit(1);
  }

  // "invalid" event fires when you have changed a file, and Webpack is
  // recompiling a bundle. WebpackDevServer takes care to pause serving the
  // bundle, so if you refresh, it'll wait instead of serving the old one.
  // "invalid" is short for "bundle invalidated", it doesn't imply any errors.
  compiler.plugin('invalid', function() {
    if (isInteractive) {
      clearConsole();
    }
    console.log('Compiling...');
  });

  var isFirstCompile = true;

  // "done" event fires when Webpack has finished recompiling the bundle.
  // Whether or not you have warnings or errors, you will get this event.
  compiler.plugin('done', function(stats) {
    if (isInteractive) {
      clearConsole();
    }

    // We have switched off the default Webpack output in WebpackDevServer
    // options so we are going to "massage" the warnings and errors and present
    // them in a readable focused way.
    var messages = formatWebpackMessages(stats.toJson({}, true));
    var isSuccessful = !messages.errors.length && !messages.warnings.length;
    var showInstructions = isSuccessful && (isInteractive || isFirstCompile);

    if (isSuccessful) {
      console.log(chalk.green('Compiled successfully!'));
    }

    if (typeof onReadyCallback === 'function') {
      onReadyCallback(showInstructions);
    }
    isFirstCompile = false;

    // If errors exist, only show errors.
    if (messages.errors.length) {
      console.log(chalk.red('Failed to compile.'));
      console.log();
      messages.errors.forEach(message => {
        console.log(message);
        console.log();
      });
      return;
    }

    // Show warnings if no errors were found.
    if (messages.warnings.length) {
      console.log(chalk.yellow('Compiled with warnings.'));
      console.log();
      messages.warnings.forEach(message => {
        console.log(message);
        console.log();
      });
      // Teach some ESLint tricks.
      console.log('You may use special comments to disable some warnings.');
      console.log('Use ' + chalk.yellow('// eslint-disable-next-line') + ' to ignore the next line.');
      console.log('Use ' + chalk.yellow('/* eslint-disable */') + ' to ignore all warnings in a file.');
    }
  });

  return compiler;
};