en.javascript.info/1-js/09-classes/01-class-patterns/1-inheritance-error-assign/solution.md

Here's the line with the error:

Rabbit.prototype = Animal.prototype;

Here Rabbit.prototype and Animal.prototype become the same object. So methods of both classes become mixed in that object.

As a result, Rabbit.prototype.walk overwrites Animal.prototype.walk, so all animals start to bounce:

function Animal(name) {
  this.name = name;
}

Animal.prototype.walk = function() {
  alert(this.name + ' walks');
};

function Rabbit(name) {
  this.name = name;
}

*!*
Rabbit.prototype = Animal.prototype;
*/!*

Rabbit.prototype.walk = function() {
  alert(this.name + " bounces!");
};

*!*
let animal = new Animal("pig");
animal.walk(); // pig bounces!
*/!*

The correct variant would be:

Rabbit.prototype.__proto__ = Animal.prototype;
// or like this:
Rabbit.prototype = Object.create(Animal.prototype);

That makes prototypes separate, each of them stores methods of the corresponding class, but Rabbit.prototype inherits from Animal.prototype.