Generate unique alphanumeric incrementing string keys

gistfile1.js

var symbols = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".split('');
var max_length = symbols.length;

for (var length = 1; length <= max_length; length++) {
  var scombos = combinations(symbols, length);
  for (var j = 0; j < scombos.length; j++) {
    var word = scombos[j];
    console.log(word.join(''));
  }
}

function combinations(superset, size) {
  var result = [];
  if (superset.length < size) {return result;}
  var done = false;
  var current_combo, distance_back, new_last_index;
  var indexes = [];
  var indexes_last = size - 1;
  var superset_last = superset.length - 1;

  // initialize indexes to start with leftmost combo
  for (var i = 0; i < size; ++i) {
    indexes[i] = i;
  }

  while (!done) {
    current_combo = [];
    for (i = 0; i < size; ++i) {
      current_combo.push(superset[indexes[i]]);
    }
    result.push(current_combo);
    if (indexes[indexes_last] == superset_last) {
      done = true;
      for (i = indexes_last - 1; i > -1 ; --i) {
        distance_back = indexes_last - i;
        new_last_index = indexes[indexes_last - distance_back] + distance_back + 1;
        if (new_last_index <= superset_last) {
          indexes[indexes_last] = new_last_index;
          done = false;
          break;
        }
      }
      if (!done) {
        ++indexes[indexes_last - distance_back];
        --distance_back;
        for (; distance_back; --distance_back) {
          indexes[indexes_last - distance_back] = indexes[indexes_last - distance_back - 1] + 1;
        }
      }
    }
    else {++indexes[indexes_last]}
  }
  return result;
}