Endlessly moving through an array backwards

You can still use the modulus operator. To go backwards, it'd be

i = (i - 1 + array.length) % array.length;

When i is 0, the partial result will be (0 - 1 + array.length), which is array.length - 1.

For any value greater than 0 but less than array.length, the modulus operator maps the value greater than array.length to the correct index in range.

Yes, you could use a single formula for both forwards and backwards by creating a common function move and pass the step as parameter.

i = (i + step + arr.length ) % arr.length;

let arr = ["One", "Two", "Three", "Four", "Five", "Six"] , i = 0
function move(step){
  i = (i + step + arr.length ) % arr.length;
  console.log(arr[i]);
}

<button id="inc" onclick="move(1)">Inc</button>
<button id="dec" onclick="move(-1)">Dec</button>