look-say-sequence.js

function lookSaySequence(num) {
  var num = num.toString().split("");
  var counters = [];
  for (var i=0; i < num.length; i++) {
    if (num[i] === num[i - 1]) {
      continue;
    }
    var current = num[i];
    var counter = 1;
    var j = i + 1
    while (current === num[j]) {
      counter += 1;
      j += 1;
    }
    counters.push(counter);
    counters.push(current);
  }
  return (counters.join(""));
}

console.log(lookSaySequence(11342556) === "211314122516") // true