What is index.js used for in node.js projects?

Other than a nice way to require all files in a directory (https://stackoverflow.com/questions/5364928/node-js-require-all-files-in-a-folder), what is index.js used for mainly?

When you pass a folder to Node's require(), it will check for a package.json for an endpoint. If that isn't defined, it checks for index.js, and finally index.node (a c++ extension format). So the index.js is most likely the entry point for requiring a module.

See the official Docs here: http://nodejs.org/api/modules.html#modules_folders_as_modules.

Also, you ask how to require all the files in a directory. Usually, you require a directory with an index.js that exposes some encapsulated interface to those files; the way to do this will be different for ever module. But suppose you wanted to include a folder's contents when you include the folder (note, this is not a best practice and comes up less often than you would think). Then, you could use an index.js that loads all the files in the directory synchronously (setting exports asynchronously is usually asking for terrible bugs) and attaches them to module.exports like so:

var path = require('path'),
    dir = require('fs').readdirSync(__dirname + path.sep);

dir.forEach(function(filename){
    
    if(path.extname(filename) === '.js' && filename !== 'index.js'){
        var exportAsName = path.basename(filename);
        module.exports[exportAsName] = require( path.join( __dirname, filename) );
    }

});

I hardly ever see people wanting to use that pattern though - most of the time you want your index.js to go something like

var part1 = require('./something-in-the-directory'),
    part2 = require('./something-else');
....
module.exports = myCoolInterfaceThatUsesPart1AndPart2UnderTheHood;

Typically in other languages the web server looks for certain files to load first when visiting a directory like / in priority, traditionally this is either: index or default. In php it would be index.php or just plain HTML it would be index.html

In Node.js, Node itself is the web server so you don't need to name anything index.js but it's easier for people to understand which file to run first.

index.js typically handles your app startup, routing and other functions of your application and does require other modules to add functionality. If you're running a website or web app it would also handle become a basic HTTP web server replacing the role of something more traditional like Apache.

Here is a good article explaining how Node.js looks for required module https://medium.freecodecamp.org/requiring-modules-in-node-js-everything-you-need-to-know-e7fbd119be8, with folder and index.js file

> Modules don’t have to be files. We can also create a find-me folder > under node_modules and place an index.js file in there. The same > require('find-me') line will use that folder’s index.js file:

~/learn-node $ mkdir -p node_modules/find-me
~/learn-node $ echo "console.log('Found again.');" > node_modules/find-me/index.js
~/learn-node $ node
> require('find-me');
Found again.
{}
>

Late to the party but the answer is simply to allow a developer to specify the public api of the folder!

When you have a bunch of JavaScript files in a folder, only a small subset of the functions and values exported from these files should exportable outside of the folder. These carefully selected functions are the public apis of the folder and should be explicitly exported (or re-exported) from the index.js file. Thus, it serves an architectural purpose.