const swap = (x, y, a) => {
  const arr = a.slice()
  arr[y] = a[x];
  arr[x] = a[y];
  return arr
}

function* heapPermutations(arr) {
  const length = arr.length
  const c = Array.apply([], { length }).map(() => 0)
	let i = 0;
  yield arr
  while (i < length) {
    if (c[i] < i) {
      arr = swap(i%2===0 ? 0 : c[i], i, arr)
      yield arr
      c[i] += 1
      i = 0
    } else {
      c[i] = 0
      i += 1
    }
  }
}

const stringPermutations = string => [...heapPermutations(string.split(''))].map(a => a.join(''))
console.log(stringPermutations('zamagni'))