code: function build(previousFileSizes) {
    console.log('Creating an optimized production build...');
  
    let compiler = webpack(Object.assign({}, config, { mode: 'production' }));
    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(
            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,
        });
      });
    });
  }
docs: Create the production build and print the deployment instructions.