I have been in this programming field for more than 4 years. I have written a lot of code. Learn different programming languages along the way from either the frontend or the backend. Sometimes it's just a little tasting and always go deep into it if I feel comfortable using it.
Now I've decided to go more towards the backend. Therefore, I often use Golang and NodeJS in accordance with my work as a backend developer.
There was also a time when I suddenly got a project idea. To make it happen, of course I need frontend skills too, although not deep. I think it's enough to make a small application.
And some honorable mentions. There are tools, architectures, best practices and more that help me when developing apps.