In mapbox.js you'll see this line: const key = {};We can use anything as a key — we could do setContext('mapbox', ...) for example. The downside of using a string is that different component libraries might accidentally use the same one; using an object literal means the keys are guaranteed not to conflict in any circumstance (since an object only has referential equality to itself, i.e. {} !== {} whereas "x" === "x"), even when you have multiple different contexts operating across many component layers.

You can use any object as the key, as Svelte uses a Map internally — in other words you could do (thing) instead of (thing.id). Using a string or number is generally safer, however, since it means identity persists without referential equality, for example when updating with fresh data from an API server.