One problem with this is that when you learn a new technology on your own, you’re likely to use patterns and idioms of the language/tech you know best, and not the appropriate ones that an experienced developer would use. When the prospective employer looks at the code, it will often seem completely naive.

As an example, when I was trying to make the transition from C and Python to C++, I wrote a moderately sized interactive, real time, visual programming language. I didn’t ever show it to anyone, but I went back and looked at it a year later: I totally facepalmed because it was filled with glaring indications that I totally wasn’t a C++ programmer. For example, I often accessed an object’s members with “this->bar” (like in Python, where you use “self.bar”).

I certainly learned a lot, so it wasn’t a waste of time (and it was a fun project for me anyway). If I had a really good friend or mentor who would have been willing to invest the time to do an extensive code review first, then it might have been a useful demo. But at the time, I had no idea just how little I knew!