permutation.js

function getStringPermutation(letters = '') {
  const lettersLength = letters.length;
  // not able to permut
  if (lettersLength < 2) return letters;
  // break letters to array
  const lettersParts = letters.split('');
  // only option to permut
  if (lettersLength === 2) {
    // reverse letters order string
    const reversedLetters = lettersParts.reverse().join('');
    // return current order and reversed order
    return new Array(letters).concat(new Array(reversedLetters));
  }
  // when lettersLength > 3
  // divide reducer to better learn
  function reducer(accumulated, letter, index) {
    // get partial string to permut
    const toPermut = letters.slice(0, index) + letters.slice(index + 1);
    // get array of permutations
    const permuted = getStringPermutation(toPermut);
    // add current letter to start of each permutation
    const toConcat = permuted.map(val => letter + val);
    // concat to array of options to return later
    return accumulated.concat(toConcat);
  }
  // execute reducer
  return lettersParts.reduce(reducer, []);
}

const toPermut = 'abc';

console.log(getStringPermutation(toPermut));