mouseover and mouseout are noisy

The mouseover and mouseout events bubble. That means with the following DOM structure...

<div id="hover-me">
    <div class="header">
        <button class="close">Close</button>
    </div>
    <div class="body">
        ... more markup
    </div>
</div>

...a mouseover subscription on #hover-me would be called three times when a user moved the mouse over the close button because

1. The user moused over #hover-me
2. The user moused over the <div> in #hover-me, and that mouseover event bubbled to #hover-me
3. The user moused over the close button, and that mouseover event bubbled to #hover-me

Chances are, you don't care about any of these except the first. That's where mouseenter and mouseleave come in. They only fire when the e.target of the event is the node subscribed to. That means no noise.

YUI().use('event-mouseenter', 'transition', function (Y) {
    var hoverMe = Y.one('#hover-me');

    hoverMe.on('mouseenter', function () {
        this.one('.header').transition('fadeIn');
    });

    hoverMe.on('mouseleave', function () {
        this.one('.header').transition('fadeOut');
    });
});

Though these events work by essentially not bubbling, you can still delegate mouseenter and mouseleave using node.delegate(...).

// The overHandler callback will be executed when any .hoverable
// element is moused over.
container.delegate('mouseenter', overHandler, '.hoverable');

mouseenter + mouseleave == hover

It's common to pair mouseenter and mouseleave subscriptions (or mouseover and mouseout for the uninitiated) to create an effect that only lasts as long as the mouse is over an element. To make that easier, the event-hover module adds a hover event. It accepts a second callback and attaches the first to mouseenter and the second to mouseleave.

YUI().use('event-hover', 'transition', function (Y) {
    function over() {
        this.one('.header').transition('fadeIn');
    }
    function out() {
        this.one('.header').transition('fadeOut');
    }

    // Two callbacks, `mouseenter` and `mouseleave`
    Y.one('#hover-me').on('hover', over, out);
});

If you need to override the default this object assignment or bind arguments to a hover subscription, just add those arguments after the second callback.

Similarly, if you want to delegate the hover event, pass a CSS filter after the second callback.

// Default `this` to racerX
mach5.on('hover', racerX.oilSlick, racerX.tireSpikes, racerX);

// The delegate filter comes before the `this` override
ruralTown.delegate('hover', ufo.scan, ufo.memoryWipe, '.person', ufo);