Strict equality check on constructor (#4985)

Author: JoviDeCroock

src/create-element.js

@@ -94,4 +94,4 @@ export function Fragment(props) {
  * @returns {vnode is VNode}
  */
 export const isValidElement = vnode =>
-	vnode != NULL && vnode.constructor == UNDEFINED;
+	vnode != NULL && vnode.constructor === UNDEFINED;

src/diff/children.js

@@ -205,7 +205,7 @@ function constructNewChildrenArray(
 				NULL,
 				NULL
 			);
-		} else if (childVNode.constructor == UNDEFINED && childVNode._depth > 0) {
+		} else if (childVNode.constructor === UNDEFINED && childVNode._depth > 0) {
 			// VNode is already in use, clone it. This can happen in the following
 			// scenario:
 			//   const reuse = <div />

src/diff/index.js

@@ -64,7 +64,7 @@ export function diff(
 
 	// When passing through createElement it assigns the object
 	// constructor as undefined. This to prevent JSON-injection.
-	if (newVNode.constructor != UNDEFINED) return NULL;
+	if (newVNode.constructor !== UNDEFINED) return NULL;
 
 	// If the previous diff bailed out, resume creating/hydrating.
 	if (oldVNode._flags & MODE_SUSPENDED) {