JAVASCRIPT

Add event listener for loop

allButtonsOnPage.forEach(function(button, index) {
  button.addEventListener('click', function() {
    logButtonIndex(index);
  });
});

using for loops to add an event listener

for(var i=0; i < elem.length; i+=2){
    var k = i + 1;
    var boxa = elem[i].parentNode.id;
    var boxb = elem[k].parentNode.id;

    elem[i].addEventListener("click", function(){makeItHappen(boxa,boxb);}, false);
    elem[k].addEventListener("click", function(){makeItHappen(boxb,boxa);}, false);
}