You’ll know an expert programmer by the quality of the code that they write. Experts have good communication skills, both sharing their own knowledge and soliciting input from others. They are self-aware, understanding the kinds of mistakes they can make, and reflective. They are also fast (but not at the expense of quality).
Experience should be measured not just on its quantity (i.e., number of years in the role), but on its quality. For example, working on a variety of different code bases, shipping significant amounts of code to production, and working on shared code bases. The knowledge of an expert is T-shaped with depth in the programming language and domain at hand, and a broad knowledge of algorithms, data structures, and programming paradigms.
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