Validation In Express-Validator

express-validator is meant to validate input passed by the browser/client; Mongoose's validation is meant to validate newly created documents. Both serve a different purpose, so there isn't a clean-cut answer to which one you should use; you could use both, even.

As for the order of validation: the checks will be performed in series. You could use async.parallel() to make it appear as if the checks are performed in parallel, but in reality they won't be since the checks are synchronous.

EDIT: node-validator (and therefore express-validator) is a string validator. Testing for uniqueness isn't a string operation but operates on your data model, so you shouldn't try to use node-validator for it (in fact, I don't even think you can).

Instead, I would suggest using Mongoose's unique feature to ensure that an e-mail address only occurs once in your database.

Alternatively, use a validator module that supports async operations, like async-validate.

Using both mongodb unique constraint and express-validator will cause a little bit complicated errors processing: you need to analyse errors in different places and translate it to common format for rest response. So another approach will be creating custom express validator which will find record with such value in mongodb:

router.use(expressValidator({
    customValidators: {
      isUsernameAvailable(username) {
        return new Promise((resolve, reject) => {
          User.findOne({ username: username }, (err, user) => {
            if (err) throw err;
            if(user == null) {
              resolve();
            } else {
              reject();
            }
          });
        });
      }
    }
  })
);

...
req.checkBody('username', 'Username is required').notEmpty();
req.checkBody('username', 'Username already in use').isUsernameAvailable();


req.asyncValidationErrors().then(() => {
...

If you need, see total example with this code on my website: https://maketips.net/tip/161/validate-username-available-using-express-validator

Conclusion: express-validator supports asynchronous validation as well so you can perform any data and string validation you need without mixing user validation and low-level db backend validation