fix: handle undefined as initial value when watching multiple values

LinusBorg · LinusBorg · commit c4748059fb01 · 2021-12-03T21:17:44.000+01:00
fix: vuejs#5032
diff --git a/packages/runtime-core/__tests__/apiWatch.spec.ts b/packages/runtime-core/__tests__/apiWatch.spec.ts
@@ -176,6 +176,21 @@ describe('api: watch', () => {
     ])
   })
 
+  it('watching multiple sources: undefined initial values and immediate: true', async () => {
+    const a = ref()
+    const b = ref()
+    let called = false
+    watch(
+      [a, b],
+      () => {
+        called = true
+      },
+      { immediate: true }
+    )
+    await nextTick()
+    expect(called).toBe(true)
+  })
+
   it('watching multiple sources: readonly array', async () => {
     const state = reactive({ count: 1 })
     const status = ref(false)
diff --git a/packages/runtime-core/src/apiWatch.ts b/packages/runtime-core/src/apiWatch.ts
@@ -295,7 +295,9 @@ function doWatch(
     return NOOP
   }
 
-  let oldValue = isMultiSource ? [] : INITIAL_WATCHER_VALUE
+  let oldValue = isMultiSource
+    ? new Array((source as []).length).fill(INITIAL_WATCHER_VALUE)
+    : INITIAL_WATCHER_VALUE
   const job: SchedulerJob = () => {
     if (!effect.active) {
       return
