Converting the array of shapes into a MultiPolygon

function shapesToMultiPolygon(shapes) {
    var allRings = shapes.map(function (shape) {
   	 // The shell of the polygon is in a separate property 'shell'.
   	 var shell = remapLinearRing(shape['shell']);
   	 // The 'holes' property is an array of linear rings for holes.
   	 var holes = shape['holes'].map(h => remapLinearRing(h));
   	 // Combine the shell and holes so that the shell is the first element in the resulting array.
   	 return [shell] + holes;
    });
    // returns an object adhering to GeoJson Multipolygon specification.
    return {
   	 'type': 'MultiPolygon',
   	 'coordinates': allRings
    };
}