solution1 - string compression

solution1.js

// if performance is not important
function compress(str) {
  /* shorten <str> by reducing consecutive character repetitions */
  return str.replace(/(.)\1*/g, function(fullMatch, g1) {
    return g1 + fullMatch.length;
  });
}

console.assert(compress('') === '', "blank string");
console.assert(compress('a') === "a1", "single char");
console.assert(compress("aaaabbaaaababbbcccccccccccc") === "a4b2a4b1a1b3c12", "long string");

// if performance is important
function compress(str) {
  /* shorten <str> by reducing consecutive character repetitions */
  var count = 0,
      lastChar = null,
      c,
      result = '';

  if (!str) {
    return '';
  }

  for (var i = 0, len = str.length; i < len; i++) {
    c = str.charAt(i);

    if (lastChar !== null && lastChar !== c) {
      result += lastChar + count;
      count = 1;
    } else {
      count++;
    }

    lastChar = c;
  }

  result += c + count;

  return result;
}

console.assert(compress('') === '', "blank string");
console.assert(compress('a') === "a1", "single char");
console.assert(compress("aaaabbaaaababbbcccccccccccc") === "a4b2a4b1a1b3c12", "long string");