text: 'use strict';
  
  // Do this as the first thing so that any code reading it knows the right env.
  process.env.BABEL_ENV = 'production';
  process.env.NODE_ENV = 'production';
  
  // Makes the script crash on unhandled rejections instead of silently
  // ignoring them. In the future, promise rejections that are not handled will
  // terminate the Node.js process with a non-zero exit code.
  process.on('unhandledRejection', err => {
    throw err;
  });
  
  // Ensure environment variables are read.
  require('../config/env');
  
  const path = require('path');
  const chalk = require('chalk');
  const fs = require('fs-extra');
  const webpack = require('webpack');
  const config = require('../config/webpack.config.prod');
  const paths = require('../config/paths');
  const checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');
  const formatWebpackMessages = require('react-dev-utils/formatWebpackMessages');
  const printHostingInstructions = require('react-dev-utils/printHostingInstructions');
  const FileSizeReporter = require('react-dev-utils/FileSizeReporter');
  const printBuildError = require('react-dev-utils/printBuildError');
  
  const measureFileSizesBeforeBuild =
    FileSizeReporter.measureFileSizesBeforeBuild;
  const printFileSizesAfterBuild = FileSizeReporter.printFileSizesAfterBuild;
  const useYarn = fs.existsSync(paths.yarnLockFile);
  
  // These sizes are pretty large. We'll warn for bundles exceeding them.
  const WARN_AFTER_BUNDLE_GZIP_SIZE = 512 * 1024;
  const WARN_AFTER_CHUNK_GZIP_SIZE = 1024 * 1024;
  
  // Warn and crash if required files are missing
  if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
    process.exit(1);
  }
  
  // First, read the current file sizes in build directory.
  // This lets us display how much they changed later.
  measureFileSizesBeforeBuild(paths.appBuild)
    .then(previousFileSizes => {
      // Remove all content but keep the directory so that
      // if you're in it, you don't end up in Trash
      fs.emptyDirSync(paths.appBuild);
      // Merge with the public folder
      copyPublicFolder();
      // Start the webpack build
      return build(previousFileSizes);
    })
    .then(
      ({ stats, previousFileSizes, warnings }) => {
        if (warnings.length) {
          console.log(chalk.yellow('Compiled with warnings.\n'));
          console.log(warnings.join('\n\n'));
          console.log(
            '\nSearch for the ' +
              chalk.underline(chalk.yellow('keywords')) +
              ' to learn more about each warning.'
          );
          console.log(
            'To ignore, add ' +
              chalk.cyan('// eslint-disable-next-line') +
              ' to the line before.\n'
          );
        } else {
          console.log(chalk.green('Compiled successfully.\n'));
        }
  
        console.log('File sizes after gzip:\n');
        printFileSizesAfterBuild(
          stats,
          previousFileSizes,
          paths.appBuild,
          WARN_AFTER_BUNDLE_GZIP_SIZE,
          WARN_AFTER_CHUNK_GZIP_SIZE
        );
        console.log();
  
        const appPackage = require(paths.appPackageJson);
        const publicUrl = paths.publicUrl;
        const publicPath = config.output.publicPath;
        const buildFolder = path.relative(process.cwd(), paths.appBuild);
        printHostingInstructions(
          appPackage,
          publicUrl,
          publicPath,
          buildFolder,
          useYarn
        );
      },
      err => {
        console.log(chalk.red('Failed to compile.\n'));
        printBuildError(err);
        process.exit(1);
      }
    );
  
  // Create the production build and print the deployment instructions.
  function build(previousFileSizes) {
    console.log('Creating an optimized production build...');
  
    let compiler = webpack(config);
    return new Promise((resolve, reject) => {
      compiler.run((err, stats) => {
        if (err) {
          return reject(err);
        }
        const messages = formatWebpackMessages(stats.toJson({}, true));
        if (messages.errors.length) {
          // Only keep the first error. Others are often indicative
          // of the same problem, but confuse the reader with noise.
          if (messages.errors.length > 1) {
            messages.errors.length = 1;
          }
          return reject(new Error(messages.errors.join('\n\n')));
        }
        if (
          process.env.CI &&
          (typeof process.env.CI !== 'string' ||
            process.env.CI.toLowerCase() !== 'false') &&
          messages.warnings.length
        ) {
          console.log(
            chalk.yellow(
              '\nTreating warnings as errors because process.env.CI = true.\n' +
                'Most CI servers set it automatically.\n'
            )
          );
          return reject(new Error(messages.warnings.join('\n\n')));
        }
        return resolve({
          stats,
          previousFileSizes,
          warnings: messages.warnings,
        });
      });
    });
  }
  
  function copyPublicFolder() {
    fs.copySync(paths.appPublic, paths.appBuild, {
  
      dereference: true,
      filter: file => file !== paths.appHtml,
    });
  }
  