allPermutations.js

// Write a recursive function for generating all permutations of an input string. 
// Return them as a set.

const input = 'abc';

const expected = new Set([
  'abc',
  'acb',
  'bac',
  'bca',
  'cab',
  'cba'
]);

console.log(getPermutations(input));

function getPermutations(string) {

  // Base case
  if (string.length <= 1) {
    return new Set([string]);
  }

  const allCharsExceptLast = string.slice(0, -1);
  const lastChar = string[string.length - 1];

  // Recursive call: get all possible permutations for all chars except last
  const permutationsOfAllCharsExceptLast = getPermutations(allCharsExceptLast);

  // Put the last char in all possible positions for each of the above permutations
  const permutations = new Set();
  permutationsOfAllCharsExceptLast.forEach(permutationOfAllCharsExceptLast => {
    for (let position = 0; position <= allCharsExceptLast.length; position++) {
      const permutation = permutationOfAllCharsExceptLast.slice(0, position) + lastChar + permutationOfAllCharsExceptLast.slice(position);
      permutations.add(permutation);
    }
  });

  return permutations;
}