offsetX

offsetX is a property of mouse and pointer events that provides the horizontal coordinate of the pointer relative to the padding edge of the element that received the event. It is typically expressed in CSS pixels and is commonly used in drawing and hit-testing scenarios, such as determining where a user clicked or moved the mouse inside a canvas element.

In practice, offsetX is defined as a coordinate on the event’s target element. This means the value depends on the element that actually handled the event, and not on the page or document as a whole. For example, if an event listener is attached to a nested child element, offsetX will be relative to that child, not to a parent container.

There are important caveats. offsetX and offsetY are not uniformly defined for all input types, and touch events do not provide them directly. Behavior can vary across browsers, and the value can be affected by page transforms, scrolling, borders, or padding on the target element. In cases involving transformed or scaled elements, offsetX may not map cleanly to a user’s intuitive coordinate.

Most developers avoid relying solely on offsetX for robust coordinate calculations. A common approach is to use clientX and clientY in combination with the element’s bounding rectangle (obtained via getBoundingClientRect) to compute coordinates relative to the element, applying any necessary scale factors. This tends to yield consistent results across browsers and input modalities. See also offsetY for the corresponding vertical coordinate.