polymer-build/scripts/generate-helpers.js

// @ts-check
const babelCore = require('@babel/core');
const path = require('path');
const fs = require('fs');
const babelPresetMinify =
    require('babel-preset-minify')({}, {simplifyComparisons: false});

/*
 * There doesn't seem to be documentation on what helpers are available, or
 * which helpers are required for which transforms. The
 * source is here:
 * https://github.com/babel/babel/tree/master/packages/babel-helpers
 *
 * You can usually tell what the helpers are used for by searching the babel
 * source to find out which plugin packages make an `addHelper` call for it.
 *
 * All helpers are listed here, with some commented out, so it's clear what
 * we've excluded.
 */
const mainHelpers = [

  // __proto__ assignment
  'defaults',
  'extends',

  // es2015 classes
  'assertThisInitialized',
  'classCallCheck',
  'construct',
  'createClass',
  'get',
  'getPrototypeOf',
  'inherits',
  //'inheritsLoose',
  'possibleConstructorReturn',
  'set',
  'setPrototypeOf',
  'superPropBase',
  'isNativeFunction',
  'wrapNativeSuper',

  // es2015 array-spread
  'slicedToArray',
  //'slicedToArrayLoose',
  'toArray',
  'toConsumableArray',
  'arrayWithoutHoles',
  'arrayWithHoles',
  'iterableToArray',
  'iterableToArrayLimit',
  //'iterableToArrayLimitLoose',
  'nonIterableSpread',
  'nonIterableRest',

  // es2015 instanceof
  'instanceof',

  // es2015 arrow-functions
  'newArrowCheck',

  // es2015 typeof-symbol
  'typeof',

  // es2015 computed-properties
  'defineEnumerableProperties',
  'defineProperty',

  // es2015 block-scoping
  'readOnlyError',
  'temporalRef',
  'temporalUndefined',

  // es2015 destructuring
  'objectDestructuringEmpty',
  'objectWithoutProperties',
  'objectWithoutPropertiesLoose',

  // es2015 template-literals
  'taggedTemplateLiteral',
  //'taggedTemplateLiteralLoose',

  // es2017 async-to-generator
  'asyncToGenerator',

  // es2018 proposal-async-generator-functions
  'AsyncGenerator',
  'AwaitValue',
  'asyncGeneratorDelegate',
  'asyncIterator',
  'awaitAsyncGenerator',
  'wrapAsyncGenerator',

  // es2018 proposal-object-rest-spread
  'objectSpread',
  'toPropertyKey',

  // proposal-function-sent
  //'skipFirstGeneratorNext',

  // proposal-class-properties
  //'classNameTDZError',

  // proposal-decorators
  //'applyDecoratedDescriptor',
  //'initializerDefineProperty',
  //'initializerWarningHelper',

  // react-inline-elements
  //'jsx',
];

/**
 * Babel helpers needed only for ES module transformation. We bundle these with
 * the require.js AMD module loader instead so that the AMD transform does not
 * depend on loading all Babel helpers.
 */
const amdHelpers = [
  'interopRequireDefault',
  'interopRequireWildcard',
];

minifyAndWriteJs(
  // @ts-ignore
  babelCore.buildExternalHelpers([...mainHelpers, ...amdHelpers]),
      'babel-helpers-full.min.js');

minifyAndWriteJs(
  // @ts-ignore
  babelCore.buildExternalHelpers(amdHelpers),
      'babel-helpers-amd.min.js');

const dir = path.dirname(require.resolve('regenerator-runtime'));
const js = fs.readFileSync(path.join(dir, 'runtime.js'), 'utf-8');
minifyAndWriteJs(js, 'regenerator-runtime.min.js');

/**
 * @param {string} js
 * @param {string} filename
 */
function minifyAndWriteJs(js, filename) {
  const minified =
      // @ts-ignore
      babelCore.transform(js, {presets: [babelPresetMinify]}).code;
  const libDir = path.join(__dirname, '..', 'lib');
  try {
    fs.mkdirSync(libDir);
  } catch (e) { /* don't care */ }
  fs.writeFileSync(path.join(libDir, filename), minified, {encoding: 'utf-8'});
}