permutations

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

• 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.prototype.length equal to 2 or 1.
• ⚠️ WARNING: This function's execution time increases exponentially with each array element. Anything more than 8 to 10 entries may cause your browser to hang as it tries to solve all the different combinations.

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] ]