Above is my sample code with contents as global variable, at what point or in which function should i create csv file.

You don't have globals in M/R. or even in any other script where you have different entry points running at different time. In my past, I dealt with this using a Script Parameters that I was setting with an array and using it again and again in my reduce. But I think N/cache module should do it better.

as a side note, if you were using ES2015+ I think you could have

return {
getInputData,
map,
reduce,
summarize
}

and hence avoid repeating yourself