Another possible solution, with usage of Array.reduce.
const arr = ["apple", "orange", "raspberry"];
const arr2 = [1, 2, 4];
const insert = (arr, item, index) =>
arr.reduce(function(s, a, i) {
i === index ? s.push(item, a) : s.push(a);
return s;
}, []);
console.log(insert(arr, "banana", 1));
console.log(insert(arr2, 3, 2))