#动态polyfill方案调研

// 添加事件
function addEvent(obj, type, fn) {
    if (obj.addEventListener) {
        obj.addEventListener(type, fn, false);
    } else if (obj.attachEvent) {
        obj.attachEvent('on' + type, fn);
    }
}

// 移除事件
function removeEvent(obj, type, fn) {
    if (obj.removeEventListener) {
        obj.removeEventListener(type, fn, false);
    } else if (obj.detachEvent) {
        obj.detachEvent('on' + type, fn);
    }
}

// 原始代码
const a = 1
const b = 2
const x = a ?? b

// 转换后代码
const a = 1;
const b = 2;
const x = a !== null && a !== void 0 ? a : b;

/* global AggregateError, CreateMethodProperty, IterableToList, Promise, Type */
(function () {
        // Based on https://github.com/es-shims/Promise.any/blob/master/implementation.js

        var identity = function (x) {
                return x;
        }

        CreateMethodProperty(Promise, 'any', function any (iterable) {
                var C = this;
                if (Type(C) !== 'object') {
                        throw new TypeError('`this` value must be an object');
                }

                var arr;
                if (Array.isArray(iterable)) {
                        arr = iterable;
                } else {
                        try {
                                arr = IterableToList(iterable);
                        } catch (_error) {
                                return Promise.reject(new TypeError('Argument of Promise.any is not iterable'));
                        }
                }

                var thrower = function (value) {
                        return C.reject(value);
                };

                var promises = arr.map(function (promise) {
                        var itemPromise = C.resolve(promise);
                        try {
                                return itemPromise.then(thrower, identity);
                        } catch (e) {
                                return e;
                        }
                });

                return C.all(promises).then(function (errors) {
                        throw new AggregateError(errors, 'Every promise rejected')
                }, identity);
        });
}());

Object.assign = require('object-assign')
// or

// Refer: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
if (typeof Object.assign != 'function') {
  // Must be writable: true, enumerable: false, configurable: true
  Object.defineProperty(Object, "assign", {
    value: function assign(target, varArgs) { // .length of function is 2
      'use strict';
      if (target == null) { // TypeError if undefined or null
        throw new TypeError('Cannot convert undefined or null to object');
      }

      var to = Object(target);

      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        if (nextSource != null) { // Skip over if undefined or null
          for (var nextKey in nextSource) {
            // Avoid bugs when hasOwnProperty is shadowed
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    },
    writable: true,
    configurable: true
  });
}

// ...
     presets: [
       [
         '@babel/preset-env',
         {
           // 支持chrome 58+ 及 IE 11+
           targets: {
             chrome: '58',
             ie: '11',
           }
         },
       ],
     ]
// ...

// 实现 @babel/polyfill 等同效果
  import 'core-js/stable'
  import 'regenerator-runtime/runtime'

/* Polyfill service v3.111.0
 * For detailed credits and licence information see https://github.com/financial-times/polyfill-service.
 * 
 * Features requested: Promise
 *  */


/* No polyfills needed for current settings and browser */