SOURCE

class Permutation {
  constructor (arr) {
    this.arr = Array.from(arr);
    this.length = 0;
    this.result = []
    this.run(0)
  }
  run (index) {
    if (index === this.arr.length - 1) {
      this.result.push(Array.from(this.arr))
      this.length ++ 
      return
    }
    for (let i = index; i < this.arr.length; i++) {
      let arr = this.arr;
      [arr[index], arr[i]] = [arr[i], arr[index]];
      this.run(index + 1);
      [arr[index], arr[i]] = [arr[i], arr[index]];
    }
  }
}

let p = new Permutation(['a', 'b', 'c', 'd']);
p.result.forEach(el => {
    console.log(el, '\n')
})
console.log(p.length);