How to remove the computational error in an array?

Question

JavaScript

Array.prototype.plusNext = function() {
            return this.map(function(el, i, arr) {
                return (
                    el + arr[i + 1]
                )
            })
        }

There is such a method, it adds the next element to each element of the array, and since the last element has nothing to take - it gives a computational error, what can be done with this?

Answer 1

You can try something like this (not a zhabist, so I can write with an error)
replace el + arr[i + 1] with
el + ( typeof arr[i + 1] != "undefined" ? arr[i + 1] : 0 )