javascript-algorithms/src/algorithms/sets/permutations/permutateWithRepetitions.js

/**
 * @param {*[]} permutationOptions
 * @param {number} permutationLength
 * @return {*[]}
 */
export default function permutateWithRepetitions(
  permutationOptions,
  permutationLength = permutationOptions.length,
) {
  if (permutationLength === 1) {
    return permutationOptions.map(permutationOption => [permutationOption]);
  }

  // Init permutations array.
  const permutations = [];

  // Go through all options and join it to the smaller permutations.
  permutationOptions.forEach((currentOption) => {
    const smallerPermutations = permutateWithRepetitions(
      permutationOptions,
      permutationLength - 1,
    );

    smallerPermutations.forEach((smallerPermutation) => {
      permutations.push([currentOption].concat(smallerPermutation));
    });
  });

  return permutations;
}