create-react-app/packages/babel-preset-react-app/dependencies.js

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
'use strict';

const validateBoolOption = (name, value, defaultValue) => {
  if (typeof value === 'undefined') {
    value = defaultValue;
  }

  if (typeof value !== 'boolean') {
    throw new Error(`Preset react-app: '${name}' option must be a boolean.`);
  }

  return value;
};

module.exports = function(api, opts) {
  if (!opts) {
    opts = {};
  }

  // This is similar to how `env` works in Babel:
  // https://babeljs.io/docs/usage/babelrc/#env-option
  // We are not using `env` because it’s ignored in versions > babel-core@6.10.4:
  // https://github.com/babel/babel/issues/4539
  // https://github.com/facebook/create-react-app/issues/720
  // It’s also nice that we can enforce `NODE_ENV` being specified.
  var env = process.env.BABEL_ENV || process.env.NODE_ENV;
  var isEnvDevelopment = env === 'development';
  var isEnvProduction = env === 'production';
  var isEnvTest = env === 'test';
  var areHelpersEnabled = validateBoolOption('helpers', opts.helpers, false);
  if (!isEnvDevelopment && !isEnvProduction && !isEnvTest) {
    throw new Error(
      'Using `babel-preset-react-app` requires that you specify `NODE_ENV` or ' +
        '`BABEL_ENV` environment variables. Valid values are "development", ' +
        '"test", and "production". Instead, received: ' +
        JSON.stringify(env) +
        '.'
    );
  }

  return {
    // Babel assumes ES Modules, which isn't safe until CommonJS
    // dies. This changes the behavior to assume CommonJS unless
    // an `import` or `export` is present in the file.
    // https://github.com/webpack/webpack/issues/4039#issuecomment-419284940
    sourceType: 'unambiguous',
    presets: [
      isEnvTest && [
        // ES features necessary for user's Node version
        require('@babel/preset-env').default,
        {
          targets: {
            node: 'current',
          },
          // Do not transform modules to CJS
          modules: false,
        },
      ],
      (isEnvProduction || isEnvDevelopment) && [
        // Latest stable ECMAScript features
        require('@babel/preset-env').default,
        {
          // We want Create React App to be IE 9 compatible until React itself
          // no longer works with IE 9
          targets: {
            ie: 9,
          },
          // Users cannot override this behavior because this Babel
          // configuration is highly tuned for ES5 support
          ignoreBrowserslistConfig: true,
          // If users import all core-js they're probably not concerned with
          // bundle size. We shouldn't rely on magic to try and shrink it.
          useBuiltIns: false,
          // Do not transform modules to CJS
          modules: false,
        },
      ],
    ].filter(Boolean),
    plugins: [
      // Polyfills the runtime needed for async/await, generators, and friends
      // https://babeljs.io/docs/en/babel-plugin-transform-runtime
      [
        require('@babel/plugin-transform-runtime').default,
        {
          corejs: false,
          helpers: areHelpersEnabled,
          regenerator: true,
          // https://babeljs.io/docs/en/babel-plugin-transform-runtime#useesmodules
          // We should turn this on once the lowest version of Node LTS
          // supports ES Modules.
          useESModules: isEnvDevelopment || isEnvProduction,
        },
      ],
      // function* () { yield 42; yield 43; }
      !isEnvTest && [
        require('@babel/plugin-transform-regenerator').default,
        {
          // Async functions are converted to generators by @babel/preset-env
          async: false,
        },
      ],
      // Adds syntax support for import()
      require('@babel/plugin-syntax-dynamic-import').default,
    ].filter(Boolean),
  };
};