Newbie to Node.js here. I'm learning Node.js via the tutorials at [NodeSchool.io](http://nodeschool.io/), and in one tutorial where we learned about modules, we were required to write something like this code:

```
// Some code...

function filteredLs(dir, ext, callback) {
    fs.readdir(dir, function(err, files) {
        if (err)
            return callback(err); // return statement necessary here...
        callback(null, withExtension(files, ext)); // ...but not here
    })
}

module.exports = filteredLs;

```

My question is, in examples like these, why is it necessary to include the `return` statement when handling the error, but OK to omit when it's null? I don't see what use the return value of the function could have to `readdir` anyhow, since it happens after it finishes its work. Why does it make a difference?