permutations

Generates all permutations of an array's elements (contains duplicates).

⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries will cause your browser to hang as it tries to solve all the different combinations.

Use recursion. For each element in the given array, create all the partial permutations for the rest of its elements. Use Array.prototype.map() to combine the element with each partial permutation, then Array.prototype.reduce() to combine all permutations in one array. Base cases are for array length equal to 2 or 1.

const permutations = (arr) => {
  if (arr.length <= 2) return arr.length === 2 ? [arr, [arr[1], arr[0]]] : arr;
  return arr.reduce(
    (acc, item, i) =>
      acc.concat(
        permutations([...arr.slice(0, i), ...arr.slice(i + 1)]).map((val) => [
          item,
          ...val,
        ])
      ),
    []
  );
};

permutations([1, 33, 5]); // [ [ 1, 33, 5 ], [ 1, 5, 33 ], [ 33, 1, 5 ], [ 33, 5, 1 ], [ 5, 1, 33 ], [ 5, 33, 1 ] ]