var str = 'let us pretend that this is a blog about gardening&cooking; here's an apostrophe & ampersand just for fun.';

console.log('Before: ' + str);


str = str.replace(/&(?:#x?)?[0-9a-z]+;?/gi, function(m){
  var d = document.createElement('div');
  console.log(m);
  d.innerHTML = m.replace(/&/, '&');
  console.log(d.innerHTML + '|' + d.textContent);
  return !!d.textContent.match(m.replace(/&/, '&')[0]) ? m : d.textContent;
});


console.log('After: ' + str);

str.replace(/&([#x]d+;|[a-z]+;)/g, "&$1")

str.replace(/&(?=[#x]d+;|[a-z]+;)/g, "&")