Returns a map of characters & their consecutive frequency.

countChars.js

// 'AAAABBCDDDEEAA' => 'A4B2C1D3E2A2'

function countChars (str) {
  var currentChar = str[0];
  var charCount = 1;
  var results = [];

  for (var i = 1; i < str.length; i++) {
    if(str[i] === currentChar) {
      charCount++;
    } else {
      results.push(str[i - 1], charCount);
      charCount = 1;
      currentChar = str[i];
    }
  }

  return results.join('');
}

console.log(countChars('AAAABBCDDDEEAA'))

I also tried using a higher function to iterate with but I before the native for loop.

function countChars (str) {
  var currentChar = str[0];
  var charCount = 1;
  var results = [];

  Array.prototype.forEach.call(str, function(char, index) {
    if(index === 0) return

    if(str[index] === currentChar) {
      charCount++;
    } else {
      results.push(str[index - 1], charCount);
      charCount = 1;
      currentChar = str[index];
    }
  })
  return results.join('');
}
console.log(countChars('AAAABBCDDDEEAA'))