Add features

Your core runs. Now you add the rest — and this is where projects quietly go wrong.

The danger isn't the new feature. It's that adding something breaks something that already worked. You add sorting, and marking-done stops working. You fix that, and adding breaks.

That has a name you already know: a regression. You met it in Chapter 6, and now you're at the scale where it actually bites.

Here's the habit that prevents it. Write down every feature that currently works — three or four lines:

✅ Add a task ✅ View tasks in date order ✅ Mark done

After every change, run through the whole list. It takes a minute.

Professionals automate exactly this and call them regression tests. Yours is a piece of paper, and it does the same job: it catches the thing you broke without noticing.