As a Full Stack Developer myself, I believe that the following are required to be one:
Develop a UI (web, mobile, or desktop)
Develop a server MVC APP in your language of choice
Data Persistence (files, databases, or some type of cache)
Those skills are essential to building a minimum viable business application. Also, there are many here who believe a Full Stack dev is a jack of all trades, doesn't really know how to do advanced programming. Know this: an excellent full stack dev understands the fundamentals of developing user friendly interfaces, the fundamentals of efficient programing and app architectures (MVC, MVVM, Flux, etc.), and unit testing. Understanding those concepts will allow you to pick up new languages, frameworks or other tools quickly. Yes, the downside is lack of mastery in the skills learned, which is why I recommend that full stack devs research which languages and frameworks have been battle tested and will be used for a long time, pick a small amount of those tools and master those.
In my case, I have learned/dabbled with 11 web frameworks, 7 programming languages, and do browser, server, and mobile development, but I will only stick with PHP, Python, and Node.js for servers, Vue and React for browsers, and React Native for some quick mobile dev (although I'm probably gonna replace that with Flutter in the future). To me those are the skills that have the most job opportunities, so I will master those.
P.S. Knowning Wordpress, and only Wordpress DOES NOT make you a full stack developer
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