Why is it expession (string) instead of a function?

[saneef]

In GeoJSON properties selection, a parameter is expression. I'm curious why is an expression (a string) preferred over a pure JavaScript function?

[neocarto]

All the parameters of the package are strings (except for data) because I make sure that this library can be easily transposed in the ecosystem of R, which is not like I JS, a purely functional language. See https://github.com/riatelab/bertin. But you are right. We have to accept functions too. That's what I started to do in the tooltips. See section 4 https://observablehq.com/@neocartocnrs/bertin-js-tooltips?collection=@neocartocnrs/bertin