Support for typed contexts

[stefanhoelzl]

Describe the problem

Currently values shared with getContext/setContext are losing their type information.

If have a simple proposal to share the types getween getContext and setContext via the used key.
So that the value returned by getContext for a certain key has the same type as the value provided with setContext for this key.

I am also willing to work on a pull request for this feature, if there is interest to have it in svelte.

Describe the proposed solution

Here is my proposed solution for the interface (similiar to vues provide/inject)

// svelte patch
import { getContext as origGetContext, setContext as origSetContext } from "svelte";

class ContextKey<T> {}
function getContext<T>(key: ContextKey<T>): T {
  return origGetContext(key);
}
function setContext<T>(key: ContextKey<T>, value: T) {
  origSetContext(key, value);
}

// define available context keys in own module
const keyA = new ContextKey<number>();

// use setContext/getContext with typing
setContext(keyA, 100); // ok
setContext(keyA, "FOOBAR"); // ts error

const n: number = getContext(keyA); // ok
const s: string = getContext(keyA); // ts error

Importance

would make my life easier

[dummdidumm]

Closing as duplicate of #8941