JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/themes/dot-luv/jquery-ui.css">
<input type="hidden" id="myField" value="1,2,3,5,8,13">

JavaScript

alert($("#myField").val());

$("#myField").val(function(i, v) {
   var arr = v.split(',');
   arr.push('21')
   return arr.join(',');
});

alert($("#myField").val());

$("#myField").val(function(i, v) {
   return $.grep(v.split(','), function(value) {  
            return value != 3;  
          }).join(',');
});

alert($("#myField").val());