There can be many things .. Mentioning few things which you can checklist for mission critical production ready applications.
- Keep your Environment variables/ Keys/ External Interfaces URL's/ Proxies IP's outside your application package. So, that if you need to change, you can manage this with no/minimum downtime for mission critical applications.
- Don't do disk operations in hot path (While the user request is processed by the server). If you need to do read data/variables stored on disk, cache them in a redis store (any) while deployment and access it from cache. Accessing disk latency can be go very high when you are handling high traffic.
- Do memory profiling and CPU profiling for your application. (Your memory usage must be constant (straight line for same load), and offload tasks which consumes more CPU cycles).
- Use static / dynamic code analyzers to identify issues with code and deviation of coding practices from standard practices. ( Dead Code / Vulnerable , Deprecated API's / Race Conditions ) etc .
- Linting your code keeps it safe and keep you informed about unwanted break points where your code may get fail. (Use Typescript/ Flow/PropTypes with front-end framework ).
There can be many things depending upon the type of application you are developing and the technologies involved.