fix(engine): add instaceof check workaround (#1165)

packages/@lwc/engine/src/framework/invoker.ts

@@ -68,7 +68,17 @@ export function invokeComponentConstructor(vm: UninitializedVM, Ctor: ComponentC
      */
     try {
         // job
-        new Ctor();
+        const result = new Ctor();
+
+        // Check indirectly if the constructor result is an instance of LightningElement. Using
+        // the "instanceof" operator would not work here since Locker Service provides its own
+        // implementation of LightningElement, so we indirectly check if the base constructor is
+        // invoked by accessing the component on the vm.
+        if (vmBeingConstructed.component !== result) {
+            throw new TypeError(
+                'Invalid component constructor, the class should extend LightningElement.'
+            );
+        }
     } catch (e) {
         error = Object(e);
     } finally {

packages/integration-karma/test/component/LightningElement/index.spec.js

@@ -1,6 +1,9 @@
 import { LightningElement } from 'lwc';
 import { createElement } from 'test-utils';
 
+import NotInvokingSuper from 'x/notInvokingSuper';
+import NotReturningThis from 'x/notReturningThis';
+
 it('should throw when trying to invoke the constructor manually', () => {
     expect(() => {
         new LightningElement();
@@ -26,3 +29,18 @@ it('should have no property enumerable on the component instance', () => {
 
     expect(enumerableProperties).toEqual([]);
 });
+
+it("should fail when the constructor doesn't invoke super()", () => {
+    expect(() => {
+        createElement('x-not-invoking-super', { is: NotInvokingSuper });
+    }).toThrowError(ReferenceError);
+});
+
+it('should fail when the constructor return an instance of LightningElement', () => {
+    expect(() => {
+        createElement('x-not-returning-this', { is: NotReturningThis });
+    }).toThrowError(
+        TypeError,
+        'Invalid component constructor, the class should extend LightningElement.'
+    );
+});

packages/integration-karma/test/component/LightningElement/x/notInvokingSuper/notInvokingSuper.js

@@ -0,0 +1,6 @@
+import { LightningElement } from 'lwc';
+
+export default class NotInvokingSuper extends LightningElement {
+    // eslint-disable-next-line constructor-super
+    constructor() {}
+}

packages/integration-karma/test/component/LightningElement/x/notReturningThis/notReturningThis.js

@@ -0,0 +1,8 @@
+import { LightningElement } from 'lwc';
+
+export default class NotReturningThis extends LightningElement {
+    constructor() {
+        super();
+        return {};
+    }
+}