most frequent character js with sets code example

Example: most popular letter in str javascript

var getMax = function (str) {
 var max = 0,
     maxChar = '';
  str.split('').forEach(function(char){
    if(str.split(char).length > max) {
        max = str.split(char).length;
        maxChar = char;
     }
  });
  return maxChar;
};