module.exports = function getCombinations(letters, count, shift = 0) {
  let variants = [];
  for (let i = shift; i < letters.length; i++) {
    if (count > 1) {
      const childCombinations = getCombinations(letters, count - 1, i + 1);
      for (let c = 0; c < childCombinations.length; c++) {
        variants.push([letters[i]].concat(childCombinations[c]));
      }
    } else {
      variants.push([letters[i]])
    }
  }

  return variants;
}

/*
Example:
getCombinations('abc', 2);

Result: [ [ 'a', 'b' ], [ 'a', 'c' ], [ 'b', 'c' ] ]
// 5 ms
*/