From c42585f0c26f6f90b4196d010e01602f545d3fd7 Mon Sep 17 00:00:00 2001
From: ersimont <8042088+ersimont@users.noreply.github.com>
Date: Sun, 1 Nov 2020 19:50:27 -0500
Subject: [PATCH] fix(app-state): unsubscribing mid-emit could sometimes result
 in an error

---
 projects/app-state/src/lib/store.spec.ts | 13 +++++++++++++
 projects/app-state/src/lib/store.ts      |  3 ++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/projects/app-state/src/lib/store.spec.ts b/projects/app-state/src/lib/store.spec.ts
index 59d1a0c9..706e6d96 100644
--- a/projects/app-state/src/lib/store.spec.ts
+++ b/projects/app-state/src/lib/store.spec.ts
@@ -157,6 +157,19 @@ describe('Store', () => {
       store('counter').set(1);
       expectSingleCallAndReset(spy, 1);
     });
+
+    it('works when the last subscriber to a child store unsubscribes mid-emit (production bug)', () => {
+      store('counter')
+        .$.pipe(skip(1))
+        .subscribe(() => {
+          sub2.unsubscribe();
+        });
+      const sub2 = store('nested').$.subscribe();
+
+      expect(() => {
+        store('counter').set(1);
+      }).not.toThrowError();
+    });
   });
 
   describe('()', () => {
diff --git a/projects/app-state/src/lib/store.ts b/projects/app-state/src/lib/store.ts
index 7bceea5b..64a46fce 100644
--- a/projects/app-state/src/lib/store.ts
+++ b/projects/app-state/src/lib/store.ts
@@ -160,7 +160,8 @@ export abstract class Store<T> extends CallableObject<GetSlice<T>> {
       }
     });
     forOwn(this.activeChildren, (children) => {
-      for (const child of children) {
+      // `children` can be undefined if emitting from a previous key caused removed all subscribers to this key
+      for (const child of children || []) {
         child.maybeEmit();
       }
     });