clone.js

const clone = (() => {
  /*! (c) Andrea Giammarchi - WTFPL */
  const toString = {}.toString;
  const flags = ['global', 'ignoreCase', 'multiline', 'sticky', 'unicode'];
  const prime = obj => typeof obj === 'object' ? new obj.constructor(obj.valueOf()) : obj;
  const through = obj => {
    const descriptors = Object.getOwnPropertyDescriptors(obj);
    Reflect.ownKeys(descriptors).forEach(key => {
      const descriptor = descriptors[key];
      if ('value' in descriptor) {
        descriptor.value = clone(descriptor.value);
      }
    });
    return Object.create(
      Object.getPrototypeOf(obj),
      descriptors
    );
  };
  return obj => {
    const asString = toString.call(obj);
    switch (asString) {
      case '[object Array]': return obj.map(clone);
      case '[object Date]': return new Date(obj.toISOString());
      case '[object Map]': return new Map(obj.entries());
      case '[object Object]': return through(obj);
      case '[object Set]': return new Set(obj.values());
      case '[object Promise]': return new Promise((res, rej) => obj.then(res, rej));
      case '[object RegExp]': return new RegExp(obj.source, flags.map(
        flag => obj[flag] ? flag[0] : ''
      ).join('').replace('s', 'y'));
      case '[object Boolean]':
      case '[object Number]':
      case '[object String]': return prime(obj);
      case '[object Null]':
      case '[object Undefined]': return obj;
    }
    switch (true) {
      case /Array\]$/.test(asString): return obj.constructor.from(obj);
      case /^\[object (?:HTML|SVG)/.test(asString): return obj.cloneNode(true);
      case /^\[object Weak/.test(asString):
        const w = new obj.constructor;
        w.get = key => w.has(key) ? w.get(key) : obj.get(key);
        w.has = key => w.has(key) || obj.has(key);
        return w;
    }
    return through(obj);
  };
})();