Javascript String charCountMap()

String.prototype.charCountMap = function (){
 var countMap = new Map()
 for (var x of this)
  countMap.set(x, (countMap.get(x) || 0) + 1)
 return countMap//from   w ww  .j  av  a 2 s . c om
}

var testStr = 'hasjdhfkjlahskdllllawuoausdjglasdkjaknxmnkfajklfjlkasjdlkfandlkfajlkdjfa'
var countMap = testStr.charCountMap()

var maxChars = []
for (var m of countMap) {
 if (m[1] > (countMap.get(maxChars[0]) || 0))
  maxChars = [m[0]]
 else if (m[1] == (countMap.get(maxChars[0]) || 0))
  maxChars.push(m[0])
}

for (var r of maxChars){
 debug('result is ' + r + ' , count is ' + countMap.get(r))
}