WheelEvent: deltaY property

The WheelEvent.deltaY read-only property is a double representing the vertical scroll amount in the WheelEvent.deltaMode unit.

You must check the deltaMode property to determine the unit of the deltaY value. Do not assume that the deltaY value is specified in pixels. Some browsers, for compatibility reasons, may return different units for the deltaY value depending on whether deltaMode has been accessed, to accommodate for websites not explicitly checking the deltaMode property.

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

console.log(syntheticEvent.deltaY);