mootools element.src

I am trying to decipher the following code

function add_img(el) {
        el.addEvents({
            'mouseenter':  function() {
           开发者_Python百科     var source = el.src;
                if (!(source.match("_active")))
                {
                    var newSource = source.replace(/_off/, "_on");
                    el.src = newSource;
                }
            }...

I was assuming el.src would change the img src of the element but instead it is accessing and changing the class, I cannot find src in documentation of mootools anywhere.. why is class getting changes instead of src?

---

src is an attribute on an HTML img element. Its the path to the image.