length-1 
First negative index through Array
var getLengthMinus1 = require('./length-1');
// Arrays fixed
var arr = [1, 2, 3];
arr[-1]; // 3
// reusable tested functionality
Object.defineProperty(
NodeList.prototype,
-1,
{get: getLengthMinus1}
);
document.querySelectorAll('#menu li')[-1];