fix: computed was not activating if its value started as undefined

src/observe.ts

@@ -27,6 +27,7 @@ export function observe<T>(
         options = reactionOrOptions;
     }
     let dispose: (() => void) | undefined;
+    let isFirstRun = true;
     const e: ObserveEventCallback<T> = { num: 0 };
     // Wrap it in a function so it doesn't pass all the arguments to run()
     const update = function () {
@@ -55,7 +56,7 @@ export function observe<T>(
         }
 
         // Call the reaction if there is one and the value changed
-        if (reaction && (e.num > 0 || !isEvent(selectorOrRun as any)) && e.previous !== e.value) {
+        if (reaction && (e.num > 0 || !isEvent(selectorOrRun as any)) && (isFirstRun || e.previous !== e.value)) {
             reaction(e);
         }
 
@@ -66,6 +67,8 @@ export function observe<T>(
         e.num++;
 
         endBatch();
+
+        isFirstRun = false;
     };
 
     update();

tests/computed.test.ts

@@ -258,6 +258,19 @@ describe('Two way Computed', () => {
         expect(obs.test.get()).toEqual(true);
         expect(obs.test2.get()).toEqual(false);
     });
+    test('Computed activates when undefined', () => {
+        const obs = observable<boolean[]>(undefined);
+        const comp = computed(() => {
+            return obs.get()?.filter((a) => !!a);
+        });
+        let observed: boolean[];
+        observe(() => {
+            observed = comp.get();
+        });
+        expect(observed!).toEqual(undefined);
+        obs.set([]);
+        expect(observed!).toEqual([]);
+    });
     test('Two way computed value is set before calling setter', () => {
         const obs = observable(0);
 

tests/react.test.ts

@@ -676,7 +676,7 @@ describe('useObserve', () => {
         expect(num).toEqual(2);
         expect(numSets).toEqual(1);
     });
-    test('useObserve with undefined never calls reaction', () => {
+    test('useObserve with undefined calls reaction once', () => {
         let num = 0;
         let numSets = 0;
         function Test() {
@@ -693,8 +693,8 @@ describe('useObserve', () => {
         }
         render(createElement(App));
 
-        expect(num).toEqual(1);
-        expect(numSets).toEqual(0);
+        expect(num).toEqual(2);
+        expect(numSets).toEqual(1);
     });
 });