What is the difference between jQuery's mouseout() and mouseleave()?

The mouseleave event differs from mouseout in the way it handles event bubbling. If mouseout were used in this example, then when the mouse pointer moved out of the Inner element, the handler would be triggered. This is usually undesirable behavior. The mouseleave event, on the other hand, only triggers its handler when the mouse leaves the element it is bound to, not a descendant. So in this example, the handler is triggered when the mouse leaves the Outer element, but not the Inner element.

Source: http://api.jquery.com/mouseleave/

There can be times when mouseout is a better choice than mouseleave.

For example, let's say you've created a tooltip that you want displayed next to an element on mouseenter. You use setTimeout to prevent the tooltip from popping up instantly. You clear the timeout on mouseleave using clearTimeout so if the mouse leaves the tooltip won't be displayed. This will work 99% of the time.

But now let's say the element you have a tooltip attached to is a button with a click event, and let's also assume this button prompts the user with either a confirm or alert box. The user clicks the button and the alert fires. The user pressed it fast enough that your tooltip didn't have a chance to pop up (so far so good).

The user presses the alert box OK button, and the mouse leaves the element. But since the browser page is now in a locked state, no javascript will fire until the OK button has been pressed, meaning your mouseleave event WILL NOT FIRE. After the user presses OK the tooltip will popup (which is not what you wanted).

Using mouseout in this case would be the appropriate solution because it will fire.

jQuery API doc:

mouseout

This event type can cause many headaches due to event bubbling. For instance, when the mouse pointer moves out of the Inner element in this example, a mouseout event will be sent to that, then trickle up to Outer. This can trigger the bound mouseout handler at inopportune times. See the discussion for .mouseleave() for a useful alternative.

So mouseleave is a custom event, which was designed because of the above reason.

http://api.jquery.com/mouseleave/