[Meta] Improve typescript support

[twelch]

Goals:

• #2158 Migrate turf-helpers/lib/geojson to @types/geojson
• Drop turf-helpers/lib/geojson.ts altogether completely, or at least no more re-exporting types from geojson.
• #2235 Should be able to use the top-level turf library directly in a Typescript project, rather than individual packages.

Related tickets:

• #1658 compatibility with @types/geojson

• #1906

• #1289 export type declaration when published build (resolved?)

• #1297 types play nice with mapbox-gl-js (this can be verified once a v7 alpha/beta release is published)

• #1504 (perhaps this can be verified once a v7 alpha/beta release is published)

[robinsummerhill]

Hi. Any progress on this? The incompatibility between turf/geojson/mapboxgl is a continual sticking point for us.

[david-morris]

I'd like to add that this library is type-incompatible with itself because of union return types. Here's an example of how to take a list of polygons and convert them to a collection of lines:

featureCollection(
    places.flatMap(
      ({ geometry, name }) =>
        (polygonToLine(geometry, { properties: { name } }) as FeatureCollection)
          .features
    )
  )

It requires a type assertion.

Presumably, that's because certain arguments to polygonToLine would cause switches between feature collections and feature output; otherwise this type should just get narrowed.

If it is because of a switch, it might make sense to make functions like this generic. A generic type could be used to restrict the allowable input options (specifically those output switches) and narrow the output type.

Generic typing would also need to be documented.

[alamenai]

Any updates. I want to use Turf Mask but it does not support TypeScript.

[smallsaucepan]

Hi @alamenai Try the latest 7.0.0-alpha.114. Typescript support is be much improved in v7.