Project: a to-do data model

The data model behind every to-do app

Strip the buttons off any to-do app and what remains is a data model: an array of objects plus a few functions that change it. That is what we build, and it runs entirely in the console.

The design:

• Each todo is an object: { id: 1, title: "Learn JavaScript", done: false }
• The id is a number that never repeats, so we can point at one todo later.
• addTodo(title) pushes a new object (lesson 4-2) and bumps the id counter.
• completeTodo(id) uses find (lesson 7-2) to locate the todo and flips its done to true.
• pendingCount() uses filter (lesson 7-1) to count what is left. Note !t.done means not done, the ! from lesson 3-1.

This shape — data plus the functions that manage it — is what engineers call the application's state: the data a program remembers while it runs. Real apps are organized exactly this way because the screen never invents anything; it only redraws whatever the state currently says. That separation is why we can build and test the entire model in the console, before any buttons exist.