WheelEvent: deltaMode property

The WheelEvent.deltaMode read-only property returns an unsigned long representing the unit of the delta values scroll amount. Permitted values are:

Constant	Value	Description
DOM_DELTA_PIXEL	0x00	The delta values are specified in pixels.
DOM_DELTA_LINE	0x01	The delta values are specified in lines.
DOM_DELTA_PAGE	0x02	The delta values are specified in pages.

You must check the deltaMode property to determine the unit of the deltaX, deltaY, and deltaZ values. Do not assume that those values are specified in pixels. Some browsers, for compatibility reasons, may return different units for the delta* values depending on whether deltaMode has been accessed, to accommodate for websites not explicitly checking the deltaMode property.

const syntheticEvent = new WheelEvent("syntheticWheel", {
  deltaX: 4,
  deltaMode: 0,
});

console.log(syntheticEvent.deltaMode);