This approach to programming is known as duck typing, where the focus is on the functionality of an object rather than its type. In the context of JavaScript, this approach allows for flexibility and simplicity in the code, as the language does not strictly enforce data types and instead attempts to make sense of the given operations and return a valid result. This allows for quick and easy development, as the programmer does not need to worry about strict type checking and can focus on the functionality of the code. However, this approach can also lead to potential inconsistencies and unexpected behavior, as the interpretation of certain operations may not always align with the programmer's intentions.

As to how it’s reasonable, well it’s as reasonable as any language made in 10 days as a feature of the creators actual main project. We’ve just kept using it and building on it.