If your program has JavaScript, then it will probably also have some data. It's a good idea to plan your data before planning your design. After all, a main goal of your user interface is to display data to your users and to enable interactions with that data. It's hard to plan a user interface without know what it will display!
You can plan your data by reading through your backlog and deciding what data is necessary for the User Stories to work. For example the user story "As a user, I can choose a city to complete my address" would require your data to include a list of valid cities. That list needs to exist before you can show it in your user interface.
Planning data for an application is a hard thing, you'll begin learning about this in the JavaScript modules. Until then you can skip this step.
Here's some markdown you can copy-paste into /planning/data.md
:
# Data
<!-- describe the data in your program
what types are there?
what fields are there?
how are the fields related?
-->
<!-- explain how the data will be used -->
<!-- explain why you planned the data the way you did -->