Add one more test - set felix.weight second time and check average. This will surely show some broken solutions.

Use Object.defineProperty to write custom setters and getters

This might have been the (only solution?) norm when this kata was created, but now that JavaScript has a proper class syntax with getters and setters, forbidding the use of modern language features is ridiculous.

SyntaxError: Identifier 'Cat' has already been declared

Why is there a Cat hidden in preloaded?