How to calculate the distance of a polyline in Leaflet like geojson.io?

Question

I am working on a map with Mapbox and Leaflet and I am supposed to let the user draw polygons and calculate and show the are of that polygon and I also need to let the user draw

Answer 1

And also that's the solution of calculate the area of circle.

else if (type === 'circle') {
     var area = 0;
     var radius = e.layer.getRadius();
     area = (Math.PI) * (radius * radius);
     e.layer.bindPopup((area / 1000000).toFixed(2) + ' km2');
     e.layer.openPopup();
}