Counts the unique characters in a string

uniqueCharsInString.js

var uniqueCharsInStringCounter = function (string) {
  var countOfChars = {}, numberCountAndUniqueChars = '', chars, uniqueChars;

  if (!string || typeof string !== 'string') {
    throw new Error('Must call method with a string as an argument.');
  }

  chars = string.split('').sort();
  uniqueChars = chars.filter(function (char, index, array) {
    return array.indexOf(char) === index;
  });

  for (var i = 0; i < chars.length; i++) {
    countOfChars[chars[i]] = countOfChars[chars[i]] ? countOfChars[chars[i]] + 1 : 1;
  }

  for (var char in countOfChars) {
    if (countOfChars.hasOwnProperty(char)) {
      numberCountAndUniqueChars += countOfChars[char] + char;
    }
  }

  return numberCountAndUniqueChars;
};