Given an object with a length property with the type a method expects, is there ever a scenario where the method is incorrect? I'm trying to wrap my head aroun

Given an object with a length property with the type a method expects, is there ever a scenario where the method is incorrect? I'm trying to wrap my head around it: if a method needs only the length property, which is an unsigned int, is there any good reason why it shouldn't accept any object that has that property? It appears at first that this is a good quality: the code is general whilst obviously avoiding any type errors, including null/undefined errors.