code: function wheelDelta(ev) {
    // Only Firefox seems to use the line unit (which we assume to
    // be 25px), otherwise the delta is already measured in pixels.
    const unitInPixels = ev.deltaMode === DOM_DELTA_LINE ? 25 : 1;
    return -ev.deltaY * unitInPixels * ZOOM_SENSITIVITY;
  }
docs: See https://github.com/d3/d3-zoom/blob/807f02c7a5fe496fbd08cc3417b62905a8ce95fa/src/zoom.js