Simple Express error handling

express-basic.js

const app = require('express')();
const fs = require('fs');

// Resolves with a list of the filenames in dir, or rejects if dir doesn't exist or if something else goes wrong.
function readFileNames(dir) {
  return new Promise((resolve, reject) => {
    fs.readdir(dir, (err, files) => {
      if (err) {
        reject(err);
      } else {
        resolve(files);
      }
    });
  });
}

app.get('/', async (req, res, next) => {
  try {
    if (req.query.dir == null) {
      throw new Error("A 'dir' query parameter must be provided.");
    }
    const results = await readFileNames(req.query.dir);
    res.status(200).send(`Files in ${req.query.dir}:\n${results.join("\n")}\n`);
  } catch (err) {
    next(err);
  }
});

app.use(function(error, req, res, next) {
  res.status(500).send(error.toString() + '\n');
});

app.listen(3000);

/* Example output:

sollux:express-error-test$ curl localhost:3000
Error: A 'dir' query parameter must be provided.
sollux:express-error-test$ curl localhost:3000?dir=foo
Error: ENOENT: no such file or directory, scandir 'foo'
sollux:express-error-test$ curl localhost:3000?dir=.
Files in .:
index.js
node_modules
package-lock.json
package.json

*/