ArangoDB with custom JS

Yes, this can be done with User Actions. Foxx was created as a more comfortable alternative and is likely a better choice for non-trivial applications. The documentation can be intimidating but Foxx services can actually be very lightweight and simple (see my other answer). If you really don't want to use Foxx for this, here's how to do it manually:

First create a virtual module in the _modules system collection:

var db = require('org/arangodb').db;
db._modules.save({
  path: '/db:/ownTest',
  content: `
    exports.do = function (req, res, options, next) {
      res.body = 'test';
      res.responseCode = 200;
      res.contentType = 'text/plain';
    };
  `
});

Then create a route that uses it:

db._routing.save({
  url: '/ourtest', 
  action: {
    controller: 'db://ownTest'
  }
});

Finally, tell ArangoDB to update its routing cache so it notices the new route:

require('internal').reloadRouting();

If you install your JavaScript module to the js/common/ or the js/server/ directory you can use the module name (e.g. myOnDiskModule) instead of the virtual module name "db://owntest" in the controller.

For smaller modules you can just define the function inline using callback instead of controller:

db._routing.save({ 
  url: '/hello/echo',
  action: { 
    callback: `
      function (req, res) {
        res.statusCode = 200;
        res.body = require('js-yaml').safeDump({
          Hello: 'World',
          are: 'you here?'
        });
      }
    `
  } 
});

Remember to always update the routing cache after changes to the routing collection:

require('internal').reloadRouting();

Note: the callback implementation in 2.8 has a bug that will be fixed in 2.8.3. If you want to apply the fix manually, it's in commit b714dc5.