Conventions for app.js, index.js, and server.js in node.js?

Even though you can call the files anything you want, there's an advantage to calling the entry point index.js or server.js

Why index.js: When you issue npm init it will set the main entry point of the module to index.js. Some people don't change it, so they end up naming their main entry point index.js. It means there's one less thing to do.

Why server.js: If your node package is not going to be consumed by another package, but rather is a stand-alone app, then if you call your main entry point server.js, then you can issue npm start and start your app. npm start will run your server.js file by default. To change this behavior, supply a start script in package.json. If a start script exists, npm start will run that script instead.

app.js is just a convention -- the only advantage to it is that some IDEs, such as Visual Studio Code will default to app.js as the entry point of a program you debug. That way when using the most common framework, Express, which creates an app.js file, "it just works"

It's pretty simple!

If your application is to be used in other applications: index.js

If your application is NOT to be used in other applications: server.js or app.js

As stated earlier the reasons being, when invoking npm start, if not defined in package.json, looks automatically for server.js. And when including another module into your application, it looks for index.js.

Extra: I also tend to only use index.js as a file name when this is automatically found somehow. This makes me aware if the file is invoked directly or indirectly.

When using npm init it makes the default index.js.

Where I work, we didn't really settle on a format, so we have some apps with index.js, some with server.js. Also, in some we have a config.js file at the root level, others are in a config folder (so require(config/config.js). We even have one where server.js is in a server folder.

The trouble comes when we want to automate our deployment process. It becomes like technical debt when we have to make a bunch of minor modifications for each service.

That said, pick a format that makes sense to you and stick with it.

In fact all are just names and you must be consistent in your own work as pointed out by @Oka in a previous answer.

The only valid point here is that modular nature of node may play an important role in your decision, as pointed out in Folders as Modules section of NodeJS documentation there are 3 ways in which a folder may be passed to require() as an argument and the second and a common one is to automatically load the index.js file from the folder, this is how a lot of NPM packages are built and as it is simple and standard according to automatically loading NodeJS feature. It seems the best choice if you are developing an NPM package.

In the end, as others pointed out, you can choose any of the three, or even another one, but stick to your decision. My decision was to always use index.js based on the mentioned fact above.