title: Is there something like mulitiSet in JavaScript?
body: <p>I know that JavaScript now has <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set" rel="nofollow noreferrer">sets</a>, but I wonder if there is something to realize the function of <em>multiSet</em>, or if there is some framework that has the functions of multiset which I really need a lot.</p>
  
  <p>Or I have to code it by myself to do the research of <em>Red-Black Tree</em>?</p>