The answer is “whatever you like”.
To make sure we’re on the same page, a frontend developer is someone who is comfortable doing user interface work – the “front end” of a software application is the part that the “user” sees. For example, a frontend dev creates your login screen, or writes code to make requests to a server for data that is displayed to the user.
A backend developer usually writes programs that run out of sight of the user, but are just as vital to the application. For example, their code accesses a data store, or implements business logic.
A fullstack developer is comfortable doing both sorts of tasks.
For context, the software being written may not necessarily be a web application that runs in the browser. Rather, it could be used to create an executable that runs on the desktop. It could be an app which is installed on a mobile device and which connects to a server in the cloud. The idea is the same: there’s a piece which is shown to the user, and a piece which is hidden.
There is an advantage to being a fullstack developer. There seems to be more demand for fullstack devs, these days. Part of that is because some managers have bought into the idea that if they hire a fullstack dev, they can have “two for the price of one”. They’re looking for a bargain.
This can work out, but there are some caveats.
The workload has to be reasonable so that a single developer can do it all in the allotted time. So if you genuinely have only enough frontend and backend work to fill a single person’s time, then it makes sense to hire one person having both skills. Otherwise, your developer may very well burn out before the job is done, leaving you in the lurch.
In addition, the fullstack dev must be skilled enough to get both kinds of work done. If they aren’t, it may take a lot longer to get the task done than if two specialists had been working on it. This is a problem if you’ve got hard deadlines! It can also mean that your “bargain” wasn’t a bargain at all. You might wind up paying the same, or more, for the same work that could have been done by two more specialized devs.
Good managers know this. That’s why there’s still plenty of demand for developers who specialize in frontend and backend work.
It’s probably best to go into fullstack development only if both frontend and backend development interest you. Unless you’re interested, it’s going to be difficult to stay motivated and learn the new skills that keep you competent and employable.
If you’re having trouble finding a dev job, and you think it’s because “only full stack devs are getting hired!”, think again. You should probably look at your job search process before you decide to train for full stack work in a panic.