getting error 'Serial' was not declared in this scope
I learned that I needed to include Arduino.h header file

apart from that I learned .ino files and .cpp files is that the .ino files transparently include particle.h for you.

The other difference is that .ino files generate forward declarations for you. This is necessary when you’ve implemented a function later in the file than when you’ve first used it. For example, if you pass the function to something like Particle.subscribe in setup() but you’ve implemented it farther down the file.