fix(reactivity): support custom Symbol.toStringTag for collections

packages/reactivity/__tests__/reactive.spec.ts

@@ -130,6 +130,35 @@ describe('reactivity/reactive', () => {
     expect(dummy).toBe(2)
   })
 
+  test('custom collection type with custom Symbol.toStringTag is handled as a collection', () => {
+    class MyCustomMap extends Map {
+      get [Symbol.toStringTag]() {
+        return 'MyCustomMap'
+      }
+    }
+
+    const myCustomMap = new MyCustomMap()
+
+    expect(Object.prototype.toString.call(myCustomMap)).toBe(
+      '[object MyCustomMap]',
+    )
+
+    const observed = reactive(myCustomMap)
+
+    expect(isReactive(observed)).toBe(true)
+    expect(isProxy(observed)).toBe(true)
+
+    let dummy: boolean = false
+    effect(() => {
+      dummy = observed.has('foo')
+    })
+
+    expect(dummy).toBe(false)
+
+    observed.set('foo', 'bar')
+    expect(dummy).toBe(true)
+  })
+
   test('observed value should proxy mutations to original (Object)', () => {
     const original: any = { foo: 1 }
     const observed = reactive(original)

packages/reactivity/src/reactive.ts

@@ -68,14 +68,26 @@ function getTargetType(value: Target) {
   if (value[ReactiveFlags.SKIP] || !Object.isExtensible(value)) {
     return TargetType.INVALID
   }
-  const type = targetTypeMap(toRawType(value))
-  let type = targetTypeMap(rawType)
 
-  // If we got INVALID but the value is actually a plain object (even if its raw type was changed
-  // by a custom Symbol.toStringTag), then force it to be reactive.
-  if (type === TargetType.INVALID && isPlainObject(value)) {
-    type = TargetType.COMMON
+  let type = targetTypeMap(toRawType(value))
+
+  // If the raw type mapping fails, we add extra checks:
+  if (type === TargetType.INVALID) {
+    // Check for collection types even if they have a custom Symbol.toStringTag.
+    if (
+      value instanceof Map ||
+      value instanceof Set ||
+      value instanceof WeakMap ||
+      value instanceof WeakSet
+    ) {
+      type = TargetType.COLLECTION
+    }
+    // Check if the value is a plain object despite a custom tag.
+    else if (isPlainObject(value)) {
+      type = TargetType.COMMON
+    }
   }
+
   return type
 }