customary.js

var customary = function (str) {
  var freq = {}; 

  var stringArray = str.split(''); 

  for (let i=0; i<stringArray.length; i++) {
    let char = stringArray[i]; 
    if (freq.hasOwnProperty(char)) freq[char]++; 
    else {
      freq[char] = 0; 
    }
  }

  var freqSorted = Object.keys(freq).sort(function (a,b) { 
    return freq[b] - freq[a];
  });
  
  var result = ''; 
  
  freqSorted.forEach(function(item) {
    result+=item;
  });

  var _index = result.indexOf('_'); 

  return result.substring(0, _index); 
}