Fixed #1796, detect undelete of undefined property

src/types/dynamicobject.ts

@@ -30,7 +30,16 @@ const objectProxyTraps: ProxyHandler<any> = {
             return target[name]
         const adm = getAdm(target)
         const observable = adm.values.get(name as string)
-        if (observable instanceof Atom) return (observable as any).get()
+        if (observable instanceof Atom) {
+            const result = (observable as any).get()
+            if (result === undefined) {
+                // This fixes #1796, because deleting a prop that has an
+                // undefined value won't retrigger a observer (no visible effect),
+                // the autorun wouldn't subscribe to future key changes (see also next comment)
+                adm.has(name as any)
+            }
+            return result
+        }
         // make sure we start listening to future keys
         // note that we only do this here for optimization
         if (typeof name === "string") adm.has(name)

test/base/proxies.js

@@ -371,3 +371,22 @@ test("predictable 'this' - 2", () => {
     expect(a.a2()).toBe(a)
     expect(a.computed).toBe(a)
 })
+
+test("1796 - deleting / recreate prop", () => {
+    let value = observable({
+        foo: undefined // if foo is something like 'abc', it works.
+    })
+
+    const events = []
+
+    const aut = autorun(() => {
+        events.push(value.foo)
+    })
+    delete value.foo
+    value.foo = "def"
+    expect(events).toEqual([
+        undefined,
+        undefined, // ideally  not, but ok..
+        "def"
+    ])
+})