Mixing ES6 classes with manual prototype manipulation can cause this to become unpredictable due to misaligned internal slots, especially when calling super or using Reflect.construct incorrectly.
ES6 classes have internal slots that are not automatically set when you manually manipulate prototypes. For example, overriding a class's prototype with a plain object can break the internal [[ConstructorKind]] slot, leading to incorrect this binding in derived classes when calling super. Similarly, using Reflect.construct or Object.create with incorrect prototype chains can cause this to refer to unexpected objects. Example: class Parent { constructor() { this.parent = true; } } const proto = {}; Object.setPrototypeOf(Child, Parent); Object.setPrototypeOf(Child.prototype, proto); // breaks internal slots; new Child() may throw TypeError.