javascript - How to switch to quoted class name -

I am trying to switch a class with javascript. This is

  class = product neutral // background color is gray   
 this  
  class = product right / / Background color is green if (trueProduct == 12) {document.getElementById ("userAnswer"). InnerHTML = "class = 'product correct'"; }   
 HTML  
  & lt; Span id = "userAnswer" class = "product neutral" & gt; & Lt; / Span & gt; Use    
 
 
, the most convenient here.  
  var Reply = document.getElementById ("userAnswer"); Answer.classList.remove ("neutral"); Answer.classList.add ('true'); Permission for   
  classlist  you  toggle  or  add  or  delete  Gives special class without changing any other.