One option would be to pop the last item, then join all the rest by commas, and concatenate with and plus the last item:
const input = ['one', 'two', 'three', 'four'];
const last = input.pop();
const result = input.join(', ') + ' and ' + last;
console.log(result);If you can’t mutate the input array, use slice instead, and if there might only be one item in the input array, check the length of the array first:
function makeString(arr) {
if (arr.length === 1) return arr[0];
const firsts = arr.slice(0, arr.length - 1);
const last = arr[arr.length - 1];
return firsts.join(', ') + ' and ' + last;
}
console.log(makeString(['one', 'two', 'three', 'four']));
console.log(makeString(['one']));