polish: raise warning when Vue.set/delete is called on invalid values (#7452)

close #7381

src/core/observer/index.js

@@ -194,6 +194,12 @@ export function defineReactive (
  * already exist.
  */
 export function set (target: Array<any> | Object, key: any, val: any): any {
+  if (process.env.NODE_ENV !== 'production' &&
+    !Array.isArray(target) &&
+    !isObject(target)
+  ) {
+    warn(`Cannot set reactive property on non-object/array value: ${target}`)
+  }
   if (Array.isArray(target) && isValidArrayIndex(key)) {
     target.length = Math.max(target.length, key)
     target.splice(key, 1, val)
@@ -224,6 +230,12 @@ export function set (target: Array<any> | Object, key: any, val: any): any {
  * Delete a property and trigger change if necessary.
  */
 export function del (target: Array<any> | Object, key: any) {
+  if (process.env.NODE_ENV !== 'production' &&
+    !Array.isArray(target) &&
+    !isObject(target)
+  ) {
+    warn(`Cannot delete reactive property on non-object/array value: ${target}`)
+  }
   if (Array.isArray(target) && isValidArrayIndex(key)) {
     target.splice(key, 1)
     return

test/unit/modules/observer/observer.spec.js

@@ -356,6 +356,18 @@ describe('Observer', () => {
     })
   })
 
+  it('warn set/delete on non valid values', () => {
+    try {
+      setProp(null, 'foo', 1)
+    } catch (e) {}
+    expect(`Cannot set reactive property on non-object/array value`).toHaveBeenWarned()
+
+    try {
+      delProp(null, 'foo')
+    } catch (e) {}
+    expect(`Cannot delete reactive property on non-object/array value`).toHaveBeenWarned()
+  })
+
   it('should lazy invoke existing getters', () => {
     const obj = {}
     let called = false