title: NodeJS detect modified files
body: <p>App loads user's text files and each of them can be changed by user.
  On app's start I want to check if any file was changed since last time.
  I think the most efficient way is to calculate checksum of each file and save to one json file.
  On app's start I will check each file checksum and compare it to data from json file
  Is there any more optimal/efficient way of doing this ? Or how exactly calculate file checksum ?</p>