Count letters in substring.

count_letters.js

/**
* Count letters in substring.
*/

function strInput(str) {
  var letterMap = {};
  var letterArray = [];
  
  for(var index = 0; index < str.length; index++){
    if(letterMap[str[index]] !== undefined){
      letterMap[str[index]].count++;
    } else {
      letterMap[str[index]] = {count : 1};
      letterArray.push({
        counter: letterMap[str[index]],
        letter: str[index]
      });
    }
  }
  
  return letterArray.reduce((res, item) => {
    res = res.concat(item.letter);
    if(item.counter.count > 1){
      res = res.concat(item.counter.count);
    }
    return res;
  }, "");
}


var testStr = "AAAAAAABBBBBBBCCCFKRBBBBBAAAC";

var outStr = strInput(testStr);

console.log(outStr);