What makes someone a senior developer is that they reliably reduce technical risk to a project and deliver on time. There are a lot of definitions for "senior"

What makes someone a senior developer is that they reliably reduce technical risk to a project and deliver on time. There are a lot of definitions for "senior" floating around, but that's what everybody really wants. The main advice I give to junior developers is that state is the core of incidental complexity. Focusing on minimizing state to the degree practical is the simplest trick I know for producing well designed systems. The only place it doesn't necessarily work is high performance code.