I have written a REST API in nodejs.
I want to send HTTP status codes on various events.
For example when data is returned I send HTTP code 200.
What is the status code to return when the input data is missing?
This isn't specific to nodejs. HTTP status codes are general -- you can read about them on wikipedia:
https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
I think you want client error code 400, which means Bad Request.