OnLeave

OnLeave refers to a callback or handler name used in user interface programming to respond when a user action causes the pointer, focus, or active element to depart from a target element. The term is commonly encountered in event-driven UI frameworks and libraries as a semantic label for the moment a user leaves the element's interactive area.

In web development the most familiar reference is the mouseleave event. The mouseleave event fires when the pointing device exits the element’s bounding box. It differs from mouseout in that it does not bubble and does not fire when moving from the element to one of its descendants. Frameworks such as React use onMouseLeave as the corresponding event handler name; some libraries may expose onLeave as an alias.

OnLeave is also used in libraries to denote exit or cleanup actions for components, such as closing a popover, terminating an animation, or unmounting a view. When related to focus, the standard event is onBlur, which fires when keyboard focus leaves the element.

When implementing onLeave handlers, developers should account for accessibility and mobile devices where hover interactions are unavailable, and consider complementary events for enter/exit transitions, focus, and gesture-based input. Clear and idempotent handlers help maintain predictable behavior across browsers and devices.