create-react-app/packages/react-scripts/scripts/build.js

// @remove-on-eject-begin
/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */
// @remove-on-eject-end
'use strict';

// Do this as the first thing so that any code reading it knows the right env.
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 measureFileSizesBeforeBuild = FileSizeReporter.measureFileSizesBeforeBuild;
const printFileSizesAfterBuild = FileSizeReporter.printFileSizesAfterBuild;
const useYarn = fs.existsSync(paths.yarnLockFile);

// 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 }) => {
      console.log(chalk.green('Compiled successfully.'));
      console.log();

      console.log('File sizes after gzip:');
      console.log();
      printFileSizesAfterBuild(stats, previousFileSizes);
      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.'));
      console.log();
      console.log(err.message || err);
      console.log();
      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) {
        return reject(new Error(messages.errors.join('\n\n')));
      }
      if (process.env.CI && messages.warnings.length) {
        console.log();
        console.log(
          chalk.yellow(
            'Treating warnings as errors because process.env.CI = true.\n' +
              'Most CI servers set it automatically.'
          )
        );
        console.log();
        return reject(new Error(messages.warnings.join('\n\n')));
      }
      return resolve({ stats, previousFileSizes });
    });
  });
}

function copyPublicFolder() {
  fs.copySync(paths.appPublic, paths.appBuild, {
    dereference: true,
    filter: file => file !== paths.appHtml,
  });
}