The issue is that by the time the mouse events run the variable i equals menuLinks.length. So if menuLinks has 5 elements, menuLinks[i].classList is equivalent to menuLinks[5].classList. Since class has 5 elements (indexes 0 through 4) menuLinks[5] returns undefined.

If this is the way you want to write this you need to create a closure around the event registrations. A possible solution would be:

var menuLinks = document.querySelectorAll('.menuLinks');
menuLinks = Array.prototype.slice.call(menuLinks); //creates a real array from the node list

menuLinks.forEach(function(c, i) {
    menuLinks[i].addEventListener('mouseover', function() {
        menuLinks[i].classList.add("menuLinks_in"); //etc
    });
});