Get all unique permutations of character strings in an array.

permutations.js

let someChars = ['A', 'B', 'C', 'D'];

/** 
  * Handle all permutations for one array item (in relation to all other array items)
  */
function permutateAgainstOneItem(arr) {
  let [first, ...tail] = arr;
  let head = [first];
  let permutations = [];

  const reduceTail = function () {
    return tail.reduce((acc, char, i) => {

      // fix the char to the left
      head.push(char);
      tail.splice(i, 1);
      // console.log(head, tail);

      // only push to the permutation array if we've arrived recursively
      // at the end of the tail array (so no dupes!)
      if (tail.length === 1) acc.push([...head, ...tail].join(''));

      // reduce recursively as long as the tail more than one item left
      if (tail.length > 1) reduceTail();

      // as you were...
      head.pop();
      tail.splice(i, 0, char);

      return acc;
    }, permutations);

  }

  reduceTail(arr);
  return permutations;
}

/**
 * Produces all char permutations for all items in the array
 */
function permutateArray(chars) {
  const allPermutations = chars.reduce((acc, curr, i, arr) => {

    const charPermutations = permutateAgainstOneItem(arr);
    acc.push(...charPermutations);

    // take first item off the arr and move it to the back
    const first = arr.shift();
    arr.push(first);

    return acc;
  }, []);

  // Remove dupes!
  return [...new Set(allPermutations)];
}

console.log(permutateArray(someChars));