No, being a polyglot who can code in 10 different programming languages and has built their own version of Jarvis does not make you a senior developer (although you are very talented). This is because a senior developer is someone who brings an incremental benefit to the table. It is someone that makes people around him or her better, and is someone that is able to have a long term vision, deliver, and consistently produce business value.
> You can know all the cool functional programming techniques in the world, but if you are unable to communicate well with your team, or you are not able to focus on delivering actual value to a customer, you are not a senior developer.
This is the important point : What value do you bring to your company/startup/business? If you are one of the few people who can bring order of magnitude (some people love the idea of 10x ) value to your company or business, you are a senior developer. Senior Developer is not just another job title, but rather, the ability to use your experiences to provide real benefit to the stakeholders.
Senior developers contribute to the developer community. Whether it be by doing tech talks, speaking at meet ups, contributing to open source, or even writing articles. They are the type of people that share information with the industry. Instead of being in a closed off box of just their working space, they go out and communicate with people in other communities which allows them to broaden their horizons. It’s kind of like travelling: The more cultures you meet and talk to, the more you understand the similarities and differences between people, and the more you start to appreciate it.