In node.js, it seems I run into the same 3 filenames to describe the main entry point to an app:

• When using the express-generator package, an app.js file is created as the main entry point for the resulting app.
• When creating a new package.json file via npm init, one is prompted for the main entry point file. The default is given as index.js.
• In some programs I have seen, server.js serves as the main entry point as well.

Other times, still, it seems as though there are subtle differences in their usage. For example, this node app directory structure uses index.js and server.js in different contexts:

app
  |- modules
  |    |- moduleA
  |    |    |- controllers
  |    |    |    |- controllerA.js
  |    |    |    +- controllerB.js
  |    |    |- services
  |    |    |    +- someService.js
  |    |    +- index.js <--------------
  |    +- index.js <-------------------
  |- middleware.js
  +- index.js <------------------------
config
  +- index.js <------------------------
web
  |- css
  |- js
server.js <----------------------------

What are the differences, if any, between these three names?