What to write first? Front then back end or vice versa..?

First of I am a codenewbie so this article is my little experience with the topic and any advice and comments are highly appreciated!

I want to talk about how I feel to write a full stack project with some missing part.

Case nr.1 - Writing full stack project with limited back end knowledge
Yes, you read it right. I started a full stack project idea a few months ago without actually knowing back end. So obviously I started to build a front end part up. Here I dont mean just the design and interactivity but also sending http request to the back end (without actually having any). As the project went forward and I learnt more and more about NodeJs back end, I started to build the back end side with receiving and sending data.

Case nr.2 - Writing full stack project with limited front end knowledge
That might sound weird, as I just stated above I know front end. This is an ongoing project where I want to build an full stack application with React front end and NodeJS back end. Now here comes the tricky part as I dont know much (yet) about React. So I started to build the back end part of the project.

What is the main difference?
It felt more natural to write front end first, because this way I always knew exactly what data is going to be sent to the back end.
Writing back end first, I need to make notes what data and data structure I expect later from the front end. And that is okay but I am little skeptic that I can really send these data later. The reason for this is probably just because my React knowledge is so small right now that I do not see how all things going to get together alter.

All in all writing back end first felt little weird, but still manageable.

So all the experienced (back end) developer out there, please tell me how I should imagine writing just back end in real life? What information you get before starting a project?