Avoid Adding Event Multiple Times

I am dynamically adding events using addEvent("keydown", function() {}); to an element. My problem is that there are times when this code gets run on the same element twice or more. The behavior becomes clunky as the function registered for that event runs a couple of times.

Is there a way for me to run the code above only once on an element? Maybe check if the event has already been added before?

Either don't use a new function each time, or use a class or something to tell you you've added it.

Not using a new function each time

MooTools' addEvent is a fairly thin wrapper on addEventListener/attachEvent, which won't add the same function twice. So if you ensure you're using the same function, you can call addEvent again without it doing anything:

// Somewhere it's created **once**
function keyDownHandler() {
    // ....
}

then:

element.addEvent("keydown", keyDownHandler);    // Adds it only if not there

Live Example:

addIt();
addIt();

function addIt() {
  $("foo").addEvent("click", handler);
}

function handler() {
  var p = document.createElement('p');
  p.innerHTML = new Date();
  document.body.appendChild(p);
}

<div id="foo">Click me</div>
<script src="https://ajax.googleapis.com/ajax/libs/mootools/1.5.0/mootools-yui-compressed.js"></script>

Remembering you've added it:

if (!element.hasClass("keydown-handler")) {
    element.addClass("keydown-handler").addEvent("keydown", function() { /*...*/});
}

Live Example:

addIt();
addIt();

function addIt() {
  var element = $("foo");
  if (!element.hasClass("click-handler")) {
    element.addClass("click-handler").addEvent(
      "click",
      function() {
        var p = document.createElement('p');
        p.innerHTML = new Date();
        document.body.appendChild(p);
      }
    );
  }
}

<div id="foo">Click me</div>
<script src="https://ajax.googleapis.com/ajax/libs/mootools/1.5.0/mootools-yui-compressed.js"></script>