Check if arr is truthy and has a length property. Use Array.prototype.length - 1 to compute the index of the last element of the given array and return it, otherwise return undefined.
const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
// Examples
last([1, 2, 3]); // 3
last([]); // undefined
last(null); // undefined
last(undefined); // undefined