From 9c03a453a06167d611a287e4ad5e12bdf3b5e49e Mon Sep 17 00:00:00 2001
From: Yunfei He <i.heyunfei@gmail.com>
Date: Sat, 5 Jun 2021 06:13:41 +0800
Subject: [PATCH] fix(reactivity): should trigger watchEffect when using set to
 change value of array length (#720)

---
 src/reactivity/set.ts          | 14 ++++++++++----
 test/v3/reactivity/set.spec.ts | 19 +++++++++++++++++++
 2 files changed, 29 insertions(+), 4 deletions(-)
 create mode 100644 test/v3/reactivity/set.spec.ts

diff --git a/src/reactivity/set.ts b/src/reactivity/set.ts
index f33032d2..669487cf 100644
--- a/src/reactivity/set.ts
+++ b/src/reactivity/set.ts
@@ -15,10 +15,16 @@ export function set<T>(target: any, key: any, val: T): T {
       `Cannot set reactive property on undefined, null, or primitive value: ${target}`
     )
   }
-  if (isArray(target) && isValidArrayIndex(key)) {
-    target.length = Math.max(target.length, key)
-    target.splice(key, 1, val)
-    return val
+  if (isArray(target)) {
+    if (isValidArrayIndex(key)) {
+      target.length = Math.max(target.length, key)
+      target.splice(key, 1, val)
+      return val
+    } else if (key === 'length' && (val as any) !== target.length) {
+      target.length = val as any
+      ;(target as any).__ob__?.dep.notify()
+      return val
+    }
   }
   if (key in target && !(key in Object.prototype)) {
     target[key] = val
diff --git a/test/v3/reactivity/set.spec.ts b/test/v3/reactivity/set.spec.ts
new file mode 100644
index 00000000..24fb1b4f
--- /dev/null
+++ b/test/v3/reactivity/set.spec.ts
@@ -0,0 +1,19 @@
+import { set, ref, watchEffect } from '../../../src'
+
+describe('reactivity/set', () => {
+  it('should trigger watchEffect when using set to change value of array length', () => {
+    const arr = ref([1, 2, 3])
+    const spy = jest.fn()
+    watchEffect(
+      () => {
+        spy(arr.value)
+      },
+      { flush: 'sync' }
+    )
+
+    expect(spy).toHaveBeenCalledTimes(1)
+    set(arr.value, 'length', 1)
+    expect(arr.value.length).toBe(1)
+    expect(spy).toHaveBeenCalledTimes(2)
+  })
+})