Heap's algorithm implementation - permutations
From a short review:


Keep the typeof check out of the recursive part, you only need to do this during the first call of the function
The fat arrow syntax is good for inline functions, in this case I would highly encourage you to use the function keyword
If nis 1, get out of the function after the push, there is no need to execute the remainder
I like verbThing for naming function, so perhaps use swapListElements and permutateList permuteList
"heap" is not a JavaScript thing, I would not use it in your naming unless you actually created a "Heap" object
newArray is not a great name, especially since most of the time it is not actually a new array, but the array that was passed to the function
I had to re-read the code that uses j, the variable is not intuitively named, and the code could use a comment there
Furthermore, the ternary statements over 3 lines are mind breaking, I would keep them on 1 line


This is my version of Heap's algorithm, with the above taken into account



function swap(list, pos1, pos2){
  const temp = list[pos1];
  list[pos1] = list[pos2];
  list[pos2] = temp;
}

//Implements Heap's permutation algorithm
//https://en.wikipedia.org/wiki/Heap%27s_algorithm
function permute(list) {

  var out = [];
  list = typeof list === 'string' ? list.split('') : list;
  permuteList(list, list.length);

  function permuteList(list, n) {
    var i;

    if (n == 1) {
      out.push(list.join(''));
    } else {
      for (i = 0; i &lt; n - 1; i++) {
        permuteList(list, n - 1);
        if (n % 2) {
          swap(list,0, n - 1);
        } else {
          swap(list,i, n - 1);
        }
      }
      permuteList(list, n - 1);
    }
  }
  return out;
}

console.log(permute('abc'));
console.log(permute('abcdef').length);