[BaseFeature] Make property "properties" modifiable

[ChristophMaskos]

Issue description

BaseFeature as a representation of a feature should have the ability to change the properties.

properties?: Readonly<Record<string, unknown>> => properties?: Record<string, unknown>

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[mbeckem]

This is a good point, but simply making the properties writable will probably not be good enough (since the changes would not be detectable by react components etc.). I think a real solution would need something like a Feature class with properties backed by the reactivity API, as an "evolution" of the BaseFeature interface.

For the time being, I can recommend two approaches:

• Instead of changing the existing feature instance, create a new one as a copy, with the updated properties. For example:

  const oldFeature = ...;
  const newFeature = { ...oldFeature, properties: { ...oldFeature.properties, updatedProperty: "foo" }};

  This will work well with react components that detect changes based on value "sameness" (mutating existing values would be a bad idea).
• If it works in your use case, simply cast away the "readonlyness". It is just a TypeScript annotation and could be ignored. For example:

  type WritableFeature = BaseFeature & { properties: Record<string, unknown> };
  
  const existingFeature = ...; 
  const writableFeature = feature as WritableFeature; // use at your own risk
  writableFeature.properties.foo = 3;