permutation.js

'use strict';

/*
  arr = [1, 2, 3]
  for each element in arr, create subArr
    for i = 1 => subArr = [2, 3]
    for i = 2 => subArr = [1, 3]

    for each subArr, get permutations
      (ex: [2, 3] => [ [2, 3], [3, 2] ])
    for each permutation in permutations, get an array started with arr[i]
      (ex: 1, [ [2, 3], [3, 2]] => [[1, 2, 3], [1, 3, 2]])
*/

var getPermutation = (arr) => {
  if (arr.length <= 1) {
    return [arr];
  }
  let output = [];
  arr.forEach((n, i) => {
    let subArr = [...arr.slice(0, i), ...arr.slice(i + 1)];
    let permutations = getPermutation(subArr);
    permutations.forEach((permutation) => {
      output.push([arr[i], ...permutation]);
    });
  });
  return output;
};

console.log(getPermutation([1, 2, 3, 4]));