compress.js

// example input 'aaaabbcccdaa'
// example output 'a4b2c3da2'

function compress(str) {
  var input = str.split('')
    , count = 1
    , ret = [];

  for (var i = 0, len = input.length; i < len; i++) {
    if (input[i] === input[i + 1]) count++;
    else {
      ret.push(count > 1 ? (input[i] + count) : input[i])
      count = 1
    }
  }

  return ret.join('');

}

console.log(compress('aaaabbcccdaa')); // >> 'a4b2c3da2'