To-do app, as one monster function and as several smaller functions
Please don't use nt and t as variable names. If you were using them in a small limited context then it wouldn't be much of an issue, but repeatedly using them across multiple lines is going to get confusing. What's wrong with newTask and task?

In the case of var nt = new Task(toDo.value, "No note yet"); it's pretty clear what nt is there, but I'm not even sure if t is a task, or a note on the task, or user input. A clear name makes code more readable, and leaves people guessing at your intentions less.

x in displayArray is even worse. Though it is a limited context, you make it seem as though it accepts arbitrary input (x doesn't specify anything) when it clearly needs something that can have a child appended.

That said I personally like the split up functions a lot more. With individual functions, it's easier to read them and see what the functions do. It's easy for steps to get lost in a big megafunction, but when each step is it's own function you can make everything more explanatory.

However the readability principle applies again. I'm not very familiar with JS, so maybe displayArray makes sense for that, but to me I can't see why a display function is appending something to the HTML. Surely it should be another append function?