Move all items in an array in either direction by one in Typescript. Does not mutate the source array, and returns a new array.

array-move-by-one.ts

function arrayMoveByOne(source: unknown[], reverse = false): unknown[] {
  const s = [...source];
  const c = [...s];

  let i = 0;

  while (i < s.length) {
    if (reverse) {
      if (s[i - 1] == null) {
        c[s.length - 1] = s[0];
      } else {
        c[i - 1] = s[i];
      }
    } else {
      if (s[i + 1] == null) {
        c[0] = s[s.length - 1];
      } else {
        c[i + 1] = s[i];
      }
    }

    i++;
  }

  return c;
}