They need to capture the common interpreter errors in the earlier lessons. So that if you try and do liz.length it doesn't come back with a reference error, but

They need to capture the common interpreter errors in the earlier lessons. So that if you try and do liz.length it doesn't come back with a reference error, but with an explanation of why that doesn't work. Making friendly error messages in the general case is a hard problem, but doing so in a restricted environment where you know the most likely errors that beginners will make (which you can data mine over the long term) is not.