Should I use JSON or SQLite for my to-do list app?

Just a heads up, when you talk about JSON objects, remember that JSON is a format for transmitting data. You actually serialize a Javascript Object into a JSON string. For learning purposes, why not try both? You could set up your app in a way that switching storage later would be easy.

Why not give both methods a spin? They can both work for your app, and it's a great learning experience to compare them.

I'd recommend going with SQLite. It's a good way to learn about databases and SQL. Plus, it'll help you understand how to manage data better in the long run.

For a small local to-do app, either way works. If it's just a quick project for learning, using AsyncStorage with JSON is perfectly fine. You can structure your data like this:
{"2025-12-06": [{"id": 1, "text": "Buy milk", "done": false}, {"id": 2, "text": "Study Kotlin", "done": true}]}
Just read and write that object, and you’re good to go. But if you’re planning to keep adding features and maybe even sync with a backend down the line, learning SQLite might be the way to go.

If you're leaning towards a database, I'd definitely suggest SQLite or even Postgres for a broader learning experience. It's widely used in the workplace.