collatz.js

;(function() {
  var collatz = function(i) {
    var list = [i];
    while (list.slice(-1) > 1) {
      var n = list.slice(-1);
      if (n % 2 === 0) {
        n = n / 2;
      } else {
        n = 3 * n + 1;
      }
      list.push(n);
    }
    return list;
  }

  console.log(collatz(3));
}());

[ 3, 10, 5, 16, 8, 4, 2, 1 ]