getting error 'Serial' was not declared in this scope I am adding Dust Sensor to my particle photon project at home,

I got this GitHub project that I want to test before implementing a final code.

I am not an expert in cpp, I wanted to modularise the dust sensor codebase to separate library so I created new dustSensor.cpp file and kept the code linked above.

but I keep getting error: 

'Serial' was not declared in this scope

'Serial1' was not declared in this scope

'DEBUG' was not declared in this scope

'DEC' was not declared in this scope dustSensor.cpp:13:57:

I do have Serial.begin(57600) called in setup() function but still get the above error
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.