Potential breaking change in upcoming Svelte version

[rmunn]

In sveltejs/svelte#6750, there is a proposed change to the StartStopNotifier type of Svelte stores, which will be a breaking change for anyone creating custom stores. See the PR for details, but the short version is that if that PR is accepted and merged into Svelte, you'll need to add an update parameter to the StartStopNotifier callback type, as follows:

diff --git a/src/runtime/store/index.ts b/src/runtime/store/index.ts
index e947fa074..afd4ccabd 100644
--- a/src/runtime/store/index.ts
+++ b/src/runtime/store/index.ts
@@ -13,7 +13,7 @@ export type Updater<T> = (value: T) => T;
 type Invalidator<T> = (value?: T) => void;
 
 /** Start and stop notification callbacks. */
-export type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;
+export type StartStopNotifier<T> = (set: Subscriber<T>, update: (fn: Updater<T>) => void) => Unsubscriber | void;
 
 /** Readable interface for subscribing. */
 export interface Readable<T> {

In your repo, the affected code appears to be in src/utils/custom_store.ts.

If you have any comments about the proposed breaking change, please come leave a comment on the Svelte PR to let me know whether it would make your life easier or harder.