When mixing ES6 classes with prototype manipulation, how can this become unpredictable?

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.