Bigger Than Neighbors

/* Write a JavaScript function biggerThanNeighbors(index,  arr) that accept a number and an integer array
as parameters. The function should return a Boolean number: whether the element at the given position
in the array is bigger than its two neighbors (when such exist). It should return undefined when
the index doesn't exist. Write a JS program that invokes the function with the sample data below
and prints the result at the console. */

"use strict";

function biggerThanNeighbors(index,  arr) {
    if (index != Number(index) || index % 1 != 0 || index < 0 || index >= arr.length) {
        return undefined + ' - invalid index!';
    }

    for (var i = 0; i < arr.length; i += 1) {
        if (arr[i] != Number(parseInt(arr[i])) || arr[i] % 1 !== 0) {
            return undefined + ' - invalid element!';
        }
    }

    if (!index || index == arr.length - 1) {
        return 'only one neighbor';
    }

    if (arr[index] > arr[index - 1] && arr[index] > arr[index + 1]) {
        return 'bigger';
    } else {
        return 'not bigger';
    }
}

console.log(biggerThanNeighbors(2, [1, 2, 3, 3, 5]));
console.log(biggerThanNeighbors(2, [1, 2, 5, 3, 4]));
console.log(biggerThanNeighbors(5, [1, 2, 5, 3, 4]));
console.log(biggerThanNeighbors(0, [1, 2, 5, 3, 4]));
console.log(biggerThanNeighbors(4, [3, 4, 5, 6, 7]));
console.log(biggerThanNeighbors(2, [4, 5.4, 6, 1, 2]));