Polyfill to add support for the global Undefined object to browsers that do not yet support it.

Undefined-polyfill.js

// Polyfill to add support for the global Undefined object to browsers that do not yet support it.
// Released into the public domain by Jacob Bloom

(() => {
  const global_ = typeof globalThis !== 'undefined' ? globalThis :
                  typeof self !== 'undefined' ? self :
                  typeof window !== 'undefined' ? window :
                  typeof global !== 'undefined' ? global : null;
  if (global_ === null) throw new Error('Cannot polyfill Undefined: unable to locate global object');
  if('Undefined' in global_) return;

  function Undefined(value) {
    if(new.target) {
      this['[[PrimitiveValue]]'] = Undefined(value);
    } else {
      return void value;
    }
  }
  Undefined.toString = function toString() { return 'function Undefined() { [native code] }'; };
  Undefined.isUndefined = function isUndefined(value) { return Object.is(value, Undefined(value)); };

  Undefined.prototype.valueOf = function valueOf() { return this['[[PrimitiveValue]]']; };
  Undefined.prototype.toString = function toString() { return 'undefined'; };
  Undefined.prototype.toNull = function toNull() { return null; };
  if(typeof window !== 'undefined') {
    Undefined.prototype.toHTMLAllCollection = function toHTMLAllCollection() { return window.document.all; };
  }

  global_.Undefined = Undefined;
})();