"Don't be dumb" “You’re smart!” http://smartquotesforsmartpeople.com/

smartquotes.js

function smartQuotes(text) {
  text = text.replace(/(^|[-\u2014\s(\["])'/g, "$1\u2018");       // opening singles
  text = text.replace(/'/g, "\u2019");                            // closing singles & apostrophes
  text = text.replace(/(^|[-\u2014/\[(\u2018\s])"/g, "$1\u201c"); // opening doubles
  text = text.replace(/"/g, "\u201d");                            // closing doubles
  text = text.replace(/--/g, "\u2014");                           // em-dashes
  return text;
}