geojson.parse ignores alt value

[idokleinman]

Given
let pointObj = {lat: 1.1, lon: 2.2, alt: 3.3, test: 'test'};
let point = geojson.parse(pointObj, { Point: ['lat', 'lon', 'alt'] });

point will equal
{ type: 'Point', coordinates: [ 2.2, 1.1 ] }
alt value has been dropped.
Why?