Draw polygon next to an another one

Question

I know an application that does what I want but it use Leaflet and I am use Mapbox-react. I am trying to, when I draw a polygon and I am closest to an another, my cursor set automatically in the boundaries of the polygon. I want to do something like this In my code my cursor know if I am in an existing polygon or if I am not. But how can I do like the picture ?