feat(ol/feature): allow generics for feature.getProperties()

[d-koppenhagen]

with this change it's possible to pass a generic type T when calling feature.getProperties<T>() and using with TypeScript.

// before: const stop = feature?.getProperties() as Stop
const stop = feature?.getProperties<Stop>()
console.log(typeof stop) // "Stop"

closes #14868

[ahocevar]

Thanks for your contribution! However, I'm failing to understand the benefit of this change. However, the result is no better than the type cast you initially had - you still have to make an assumption on what the return type is, and TypeScript won't help you checking it because of the @ts-ignore.

I don't think there is a good easy alternative. A more involved alternative would be to define a generic PropertyType on the Feature constructor, and allow setting it also on VectorSource and FeatureFormat. If set on FeatureFormat, VectorSource should pick it up. Doable, but not easy.

[github-actions]

📦 Preview the website for this branch here: https://deploy-preview-15496--ol-site.netlify.app/.

[d-koppenhagen]

@ahocevar you were totally right. I was on the wrong path I think. Please review again. Now it should be possible to infer the type.

[ahocevar]

@d-koppenhagen Are you able to benefit from this? With your last changes, it looks like you only would if you created RenderFeature instances manually, instead of retrieving them from a FeatureFormat or a VectorTile source.

[Guillaumedemoff]

I believe the get function should also be typed accordingly.

Something like this should do the trick.

  /**
   * Get a feature property by its key.
   * @template {keyof T} Key
   * @param {Key} key Key
   * @return {T[Key]} Value for the requested key.
   * @api
   */